If you need to tell whether a mouse is male or female, gently pick up the mouse, holding it so its rear end is facing you and at least 2 of its feet are on a sturdy surface. Lift the base of the mouse’s tail so you can see its genitals. If the mouse’s genitals are about 1/4 of an inch away from its anus, the mouse is probably a female. If the genitals are much farther away from the anus, or you can see descended testicles, the mouse is a male. Also, female mice have nipples, and male mice do not.