One of the earliest decisions you will need to make when you decide to buy some rats is what sex you want. Male rats are called bucks, females are called does. There are differences between the sexes, but both make great pets.

Males
Bucks are bigger than does, sometimes even twice the size, and their fur is generally a little rougher. Bucks tend to mark their territory by leaving tiny drops of urine on whatever they wish to claim as theirs, and this will include you! Some bucks mark more than others. Some don't mark at all. Male rats tend to bond to people more than the females and are more likely to want to sit on your lap and watch TV. They can be thoroughly lazy, especially once they reach about 18 months old. Bucks are harder to introduce to new adult rats, but it is entirely possible. Male rats are less easy to do things like agility with since they don't see the point in wasting energy unless it's to go and get food! A buck will be the big, lazy lap pet who licks your hand and sleeps beside you.

Females
Does are smaller and sleeker than bucks and tend to be more active. They love to explore everything. They are prone to mammary tumours which are usually benign, and thusly harmless, but can get expensive to keep having removed. Does come into heat every 4 days or so for around 12 hours when they may become more skittish and may dislike being handled. Does scent mark a lot less than males but are also less likely to establish a toilet corner and stick to it, which makes them a little more difficult to clean. A doe will be the rat who sits on your shoulder or uses your body like a climbing frame and explores everything on your desk!

Which sex you chose is totally down to personal preference. If you want a big, laid back, squishy rat who will be content to sit on your lap then go for a buck. If you want an interesting and active pet, then go for a doe. There are always exceptions to the above. Some bucks will act more like does, and some does will behave more like bucks! The above descriptions are general guides only.

It's extremely easy to tell the difference between the sexes. Females have nipples and males don't. Male rats are also very well endowed, so theres no mistaking who is who when you put them beside each other! But here is a reference picture: