Female rats become sexually mature at about 65 days of age. They have an estrous cycle of about every 4 to 5 days throughout the year. Breeding usually occurs at night during a 12 hour period of receptiveness to the male. They also have a fertile postpartum estrus, and can be bred within 48 hours after giving birth. However, unlike mice, this usually does not occur because the male should be removed from the cage prior to the female giving birth to avoid injury to the pups by the male.
Pregnancy lasts about three weeks in rats as well.
Litter size averages 6 to 12, but smaller litters are common as described for mice. The female and her litter should not be disturbed for the first few days after birth, because a stressed female rat may injure or destroy her pups. Rats are weaned at about three weeks of age. The female will resume her normal estrous cycle within 2 to 5 days after the young are weaned. The organism is difficult to isolate by standard laboratory culture procedures.