Determine the age of your mice. Mice will usually not breed if they are younger than 4 weeks old. Similarly, mice who have been housed alone or in pairs (with the same sex) will usually not breed if they are older than 6-8 months old. On occasion, older mice will breed successfully, but the females can also have significant problems with delivering live pups, and nursing them appropriately. As a suggestion, if housing mice long term where there is a chance that you may need to breed them in the future (such as to keep the line going), you will want to breed them at least once prior to 6 months of age. The pups from this breeding may be sacrificed if not needed. This will help when future breeding is attempted. Remember, old mice are a lot like old people. As they age, the quality and quantity of both sperm and eggs decrease. Not only are there physiological changes, but there are temperament changes as well. They can become grumpy old mice.

• * Minimum breeding age:
• males: 35-42 days
• females: 21 days *Maximum age for first breeding:
• males and females: 6 months

Set up breedings only when you know that the mice will be supervised carefully during the next day. On occasion, the mice will fight after breeding, and can inflict serious wounds that could be fatal without prompt attention. Thus, mice should not be bred on Fridays or Saturdays or on Sunday on a long weekend. To breed, put the female into the male's cage. Reversing this order can result in the male killing the female (or, on rare occasions, the female killing the male). If it is not possible to put the female into the male's cage, use a clean cage, and put the male in the cage first. If this can be done one week in advance of the anticipated mating, this will allow the male to mark his territory, and the pheremone level to rise, which will aid in the breeding process.