- 1). Pick up the baby mouse, but be very gentle. Hold its body in one hand, and grab its tail with your other hand. It doesn't matter whether the mouse is right-side-up or upside-down because you can see what you need to see either way.
- 2). Lift the tail so that you can easily see the mouse's anus and urinary tract opening. In baby mice, these will look like two small dots under the tail.
- 3). Check to see how far apart the two openings are. In female mice the anus and the urinary tract opening are quite close together. In male mice, the urinary opening is much farther from the anus. The difference is very obvious when you compare a male and a female mouse, so check several mice if you aren't sure. You can see these differences as soon as the mice are born.
- 4). Look for the presence of nipples on the mouse's underside. Females that are older than 10 days old have nipples you can see, although you may need to brush her fur out of the way. Males will not have any, no matter how old they are.
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