It's very easy to tell the difference between a male and a female rat. Often people can't tell the difference until they've owned a few rats. The distance from the anus to the genitals is far greater in the male than the female. I hope to get infant photos added here in the near future, because it's much more difficult to see these differences in babies than in adults, unless you have a whole litter to look at so that you can compare the babies. It's harder too to try to compare a growing baby to an adult. Now take a look at the photos below.

Notice how the penis on the male is far away from the tail, high up nearer the belly. There are no nipples on males. See how the testicles look from behind in the second male photo. With the first male photo look closely where the fur stops, you can see two little lumps just below that fur. No matter what angle you look at them, there is no mistaking those huge testicles. Baby boys will start to drop testicles at around three to four weeks of age. You should be able to tell for certain which is which by the time they are about four to five weeks of age.

Notice on the female that you can clearly see nipples, they look like a series of dark colored dots. You should see nipples on baby girl rats by the time they are ten days or so old, long before they are weaned. See how close to the tail the female's vulva is. What appears to be "something" hanging down from her genital area is nothing but fur. You don't see any lumps of any kind.