It is an extremely good idea to keep rats purely as pets for at least couple of years before you venture into breeding, to give you an idea of the time and money investment they require.
I would also recommend joining a regional club such as The Midlands Rat Club, the UK's national club (The National Fancy Rat Society), or both, attending rat shows and becoming active on club forums so that your name and your face become known and you have a ready source of advice and support. Get to know the people, the varieties of rats, and what your aim is in breeding.
Your first task is to work out what you are trying to achieve by breeding. Do you want to create perfect pet rats? Do you want to make a particular colour or marking and win shows? Do you want to make very healthy and long lived rats? Or are you like most of us and you'd like to do all of these things at once?
Now you need to find the perfect rats to begin your line with. If your hope is to breed from a rat you already own, you need to consider its history. If your rats are from a breeder, talk to the breeder and ask for details of the parents and littermates, how their health and temperament has been, and ask for permission to breed on from the rats. If your rats are from a rescue or a petshop, it's inadvisable to use them to begin a line as you don't know what problems they may carry. Many rescue centres will have asked you to sign a no breeding contract at the point of homing.
If you don't already own the rats you need to achieve your breeding aims, now is the time to begin networking within the rat fancy. Find out who breeds rats with the pet qualities you most value, the variety you would like to breed, or who is breeding towards the same lifespan goals. Ask them to keep you in mind for kittens when they have their next litters. Bring in males first, then females, and more of each than you think you will need so that you have a choice when the time comes to mate up. These rats are the beginning of something great, so they have to be the right ones.
Naming your rattery is a useful way to identify your rats. In the UK ratteries use a prefix, a name that comes before the individual rat name, to identify which breeder the rat was bred by. Try to pick a name that means something to you and will stay relevant in coming years. It's also a good idea to do a web search to see if any other ratteries are using the name, to save confusion in the future.
There is no obligation to register your rattery name in the UK, and many perfectly valid and ethical breeders are not registered in this way. Having said this, the National Fancy Rat Society keeps a register of rattery names which is useful to allow you to see what names are already in use and to let you reserve a name for your own rattery. You will need to be a member for one year before you can register a name. Registering your rattery also allows you to begin working towards a 'stud name', achieved by winning at shows.
Opinions on the ideal age to mate your does are incredibly varied. Some lines are better mated sooner because the does put weight on early and become less fertile. Some lines are better mated up later because the does are slow to mature. Ask the breeder who bred your does what age they are best bred from.
I generally mate up my does at between six months and a year old, usually closer to a year. This is because I like to give them time to play and enjoy their lives before they have the stress of bringing up babies, and because it allows me to exclude any that have health problems before this age. There is also a theory that does that are fertile to an older age are likely to live longer, so I prefer to leave them longer and take the disappointment if they will not breed.
Bucks are better left a little longer, usually past one year old, to allow any hormonal aggression to show up. This can be hereditary so I would avoid breeding from an aggressive buck.
Female rats generally come into season once every four or five days, and once mated the gestation period is 21 to 24 days, most commonly 22.5 days but varying between lines. The day of mating is day 0.
Many breeders mate up two pairs at once, to provide a backup in case of problems. Rat mothers are very maternal and invariably willing to foster babies. An alternative is to coordinate your litter with another breeder who lives nearby.
There are two common ways of pairing rats for breeding. The first is to wait until the doe is in season, and then place her in a small cage with the buck overnight. If your doe isn't having obvious seasons, you can test the pair each evening. It will be obvious when she is ready for mating as she will freeze in position waiting for the male. If she's karate kicking him instead, she's not ready yet.
The second way of pairing rats is to simply house the doe and buck together and wait until she begins to look pregnant, then remove the buck. I don't use this method as it can be difficult to reintroduce the buck to his cage group afterwards.
A few days before their due date, many does become unsettled and more aggressive than usual with their cagemates. If this happens, you need to make the decision whether or not to move her to a birthing cage now or wait a few days. If she seems fine, then wait until day 20 or 21 to move her into a smaller cage with narrow spaced bars to await her litter. Most does take birthing in their stride and it's better to just leave her to it, as checking too much can make you both nervous. Mum will birth each baby separately, eat the placenta and cord and then clean the baby thoroughly before tucking it underneath her.
Phantom pregnancy - A doe mated to an infertile buck may show all the signs of being pregnant, only to start shrinking again after day 17 or so.
Reabsorption - rats are able to 'change their minds' about being pregnant and may reabsorb the litter right up to the last minute. If this happens, keep an eye on your doe as there may be babies that are not fully absorbed and in this case she may develop an infection that requires antibiotic treatment.
Late birth - if the litter goes past its due date there is still hope, but keep a very close eye out for infection. Rats can delay the litter for up to ten days or so if they are very stressed, but this is unusual in a well-nourished doe with a planned litter.
Stillbirth - it is not unusual for individuals or even a whole litter to be stillborn. You may not even see the babies, as mum's instinct is to 'clean up' and eat the bodies.
Stuck babies - if a baby gets stuck in the birth canal it can be very tiring for the mum, and she is not likely to be able to feed any of the babies she already has. You will need to consult your vet, and may have to have your doe spayed. If you have a foster mum handy, any babies already born should be fostered over, and any babies removed by caesarean may possibly survive to be fostered.
Lack of milk - newborn rats have very transparent skin and it is possible to see the milk in their stomachs if mum will let you. If they have not been fed, there is a chance the doe will settle to feed them, she may need moving to a carrier or small cage to encourage her, or they may be better fostered. This is a difficult decision and one that needs to be made on a case by case basis.
Dying in the nest - babies quite often 'disappear' from the nest, even up to a couple of weeks old. Usually this is because they have died and mum has 'cleaned up'. It's normal and something you just have to be prepared for.
Scattered babies - Some mums are very upset by the birth and scatter their babies instead of caring for them. Move her and the babies into a small carrier or hamster cage until the nurturing instinct kicks in.