Rats and mice are polyoestrous and have no specific breeding season. Once bred, mammary development normally occures within 15 to 16 days. A few days before the young are born, a flury of nest building will occur resulting in large nests. The nest should be left undisturbed for 2-3 days after birth. The babies, or pups, are blind, hairless, and immature at birth. They develop rapidly, however, and may be weaned at three weeks.
age | period | |||||
| Rats | 8-10 weeks | 10 weeks | 4-5 days | 20-22 days | 6-16 | 21-28 days |
| Mice | 6-7 weeks | 7 weeks | 4-5 days | 19-21 days | 8-12 | 21-28 days |
