It can be very difficult to determine the sex of a young rabbit. Although the explanation and diagram below will help you, it is best to have an experienced person show you how to sex rabbits of different ages.
To determine the sex of a rabbit:
Hold the rabbit on his/her back on your lap with the hind legs facing away from you. With a larger rabbit, you may want to hold the rabbit with his/her hind legs facing towards you. Placing a rabbit on his/her back will put the rabbit into a state almost like a trance. Rabbits still feel pain while in this position, and should not be held this way for a prolonged period of time.
When first learning to sex a rabbit, it will be helpful if another person gently restrains the head of the rabbit while you use both hands to part the fur and apply gentle pressure on each side of the vent, which is the area including the anus and the genitals. (The anus is basically the opening of the intestines to the outside from which droppings are eliminated. The genitals are the external portions of the reproductive tract.) Once you have more experience, you can restrain the rabbit's head with one hand, and apply the gentle pressure with the other.
While applying pressure to the vent area, you will see the anus, the opening closest to the tail.
The opening farthest from the tail is the genitals.
Males: In male rabbits, the penis will appear as a tubular protrusion. It is round in diameter, very light pink in young males, and has a rounded tip with a small round opening at its center. In most bucks (male rabbits) older than 10 weeks, you will also notice the testicles on each side and slightly cranial to the penis. They will feel like small mounds under the skin, and in older males, are not covered with fur. Bucks can withdraw the testicles into the abdomen, so even if you can not feel the testicles, the rabbit may still be a male.
Females: When the gentle pressure is applied to the vent area of a doe (female rabbit), you will see a pink protrusion, but the protrusion is slanted, more oval, and has a slit versus a small round opening.
Other sexual differences in older rabbits
As rabbits age, more physical differences between the sexes will become apparent. Bucks have blockier heads and are smaller than does of the same breed. Most adult does of medium or large breeds will have a dewlap, which is the large fold of skin under their chins. Does have nipples, whereas bucks do not. However, the nipples may be difficult to find on a doe that has not had a litter. So, similar to the testicles in the males, even if you can not find the nipples on a rabbit, the rabbit could still be a doe.