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Mice can be prolific breeders, but they don't become sexually mature until they are six to eight weeks old. Don't expect them to mate before this time. Just feed your mice well and keep them comfortable until they are old enough to breed. Generally, putting one male with two females in a single cage will increase the chances of breeding. To prevent fighting, it’s important that there’s only one male in each cage.  If you’re not sure which mice are male, gently lift them up by the tail and look between their legs. The testes should be very visible on male mice. Most mice will not normally fight with others. However, some male mice might become hostile, especially if they sense females nearby that are ready to mate. More rarely, some females may become aggressive with each other. If you have any aggressive mice on your hands, isolate them. Gently grasp them by the tail and move them into another cage to live by themselves instead of attempting to use them for breeding. If mice are healthy and grouped properly, there’s a good chance they will breed without any intervention. Female mice will enter estrus (be ready to breed) every three to four days, so things might happen quickly. If you are trying to breed quickly, you can rotate a couple of different females in each male mouse’s cage every 1-2 weeks. This will likely increase the number of females that become pregnant. When mice have mated, there will be a “plug” visible on the rear of the female. This plug is actually hardened semen, and it should remain in place for about 12 hours after mating. Even if you see the plug, however, it does not guarantee that the mouse has conceived and is pregnant, only that it has mated with a male.
Wait six to eight weeks before attempting to mate your mice. Decide which mice will be roommates. Isolate aggressive mice. Let the mice do their thing. Check if mating has occurred.