Selecting A Rat
When selecting a rat, first of all look at all the rats in the cage and the conditions they are kept in. They should be in good healthy condition and not be kept in overcrowded cages. The cages themselves should be clean and the rats should have access to food and water. The rats should be separated by sex with males in one cage and females in another. If buying from a pet shop and the shop staff cannot guarantee the sex of any rat this is not a good sign and buying could mean buying a pregnant female - and problems.
The rats should ideally be 5-6 weeks of age - rats should not be sold younger than this.
Once you have found a good pet shop or breeder inspect the rats carefully. If one rat appears sick the others may also be at risk of being ill so it is best to look in another cage or another shop.
Check that the rat is bright eyed and alert when it is awake. It should be inquisitive and not too nervous. Beware of any rat that has runny or sticky eyes, runny nose, sneezing, wet or dirty bottom, matted fur, seems lethargic or does not have a firm body. All of these things can indicate a sick rat.
Once you have seen a healthy rat that you like, ask if you can handle it so that you can see how tame it is. If a shop does not allow you to handle the rat you have no opportunity to assess how suitable it is as a pet and cannot check its health properly before buying so is best avoided.