The rat is the easiest pocket pet to breed. Unlike a female hamster or gerbil, which may be aggressive toward a potential mate if she is not in estrus (or heat), a female rat will simply treat her suitor platonically until the time is right. The male rat's presence may even stimulate the female to become more reproductively "ready" – she may experience a hormonal response to scent cues that the male sends out in her presence.
A female rat is receptive to a male rat's advances every 4 or 5 days. There is a distinct window of time within the 5 day estrus cycle when her mate's sperm will be capable of fertilizing her eggs. Only then will a female rat allow a male to mate with her. You can breed one male rat with one female rat or set up a harem situation in which you keep one male rat with up to six females. Keep in mind, however, that each female rat will have a litter of between 6 and 12 babies and in a harem situation, this can mean in a short time you will have lots and lots of babies. If you cannot find homes for all of the young, be aware that rats sold to pet stores often end up as snake food.
The gestation period for rats is about 3 weeks. If you've kept your rats in communal cages, separate the pregnant female or females into their own cages about a week before their babies are due. During the last few days before she gives birth, your rat will spend more time building her nest and may eat more. You shouldn't need to adjust her diet; just make sure she always has enough water and food.
The female rat delivers her babies standing on her hind legs. She quickly licks away the membrane that coats the babies at birth, severs their umbilical cords with her teeth and eats the placentas. The whole process takes less than 1 1/2 hours. Generally speaking, rats are good mothers with a low incidence of cannibalism. A lactating rat may even act as an adoptive parent to rat babies orphaned by death of another female in their colony. For the first week after the babies are born, try not to bother your rat and her litter. If her nest is disturbed, a female rat will carry her babies in her mouth one by one to a new location. This behavior is called "retrieving."
For the first week, the baby rats (which weigh less than a quarter of an ounce at birth) have closed ears and eyes. Their nervous system develops rapidly, and by 11 days, they respond to noises around them. At 2 weeks their eyes are open, and the babies are usually weaned by the time they're 3 weeks old. Puberty for rats occurs at around 50 to 50 days of age, and you should consider your new litter sexually mature by their 4th to 6th week but this will vary. Some rats have been able to breed by 4 weeks of age and others wait until they are over 8 weeks of age. Remember that female rats can be impregnated again only 48 hours after giving birth. If you'd like to breed your female again, it's best to wait a couple of weeks after she weans the first litter so that she can regain her strength.