The rat should be firm fleshed and not soft, fat or flabby, neither should you be able to feel the backbone or other bones. The coat should be tight and not loose or baggy. The coat should also be smooth and glossy, with the exception of rexoids, and should have no holes, scars or bald patches. The eyes should be bright and bold. The rat should not sit huddled; there should be no sign of the rat being tucked up. The tail shall not be scabby or the scales in poor condition. The tail should not be square. On the other hand, rats should not be fat either.
Ensure that they are fed the right balance of fat, protein and carbohydrates. During primary growth, rats require around 5-6% of dietary fat and 23-28% of dietary protein, as do breeding does. You can feed them as much as they will eat until they are ~12 weeks old, after which you can level off their feed. As adults, 10-14% of dietary protein will suffice.
Be mindful of any bullying in the group. This may cause a rat to look run down, or in some cases other rats may cause coat damage through scratches, bites, chewing and/or barbering. Also look out for skin diseases such as staph or ringworm. Staph can be treated with leucillin. Ringworm can be treated using an anti-ringworm spray. Bullying may be resolved by re-evaluating the group the rat is in through reintroductions, splitting the group, keeping them in smaller groups, and/or neutering.
Showing and breeding can lead to a temporary loss in condition. This is more pronounced in the case of over-showing and -breeding.
During nursing, does may lose a considerable amount of weight. It's very important that does are given enough time to regain any lost condition before being mated again or returning to the show bench. However, if you give them too long they may not get pregnant again. Really any doe should be back in condition within 8 - 10 weeks from having their litter, if they have not then they should not be bred again.
Exhibitors can be a bit blind to condition lost due to showing as it can be gradual. Judges can tell if a rat that they saw a week or two before on the bench is losing condition but exhibitors often don't notice. The rat tends not to look as firm, its coat not as tight and glossy. Sometimes the rat appears rather jaded. It's the norm for rats taken to shows to lose 10% of body weight during the show day or if travelling, so it's common sense to realise that the number of times a rat is shown will affect it. A lot of this weight loss is water, so you may like to consider a water bottle for all shows.
Apart from diet, rats on a show day may require nesting material, especially in the colder months. Many rats benefit from being able to nest.
A rat should not be shown twice within 6 days, and within 13 days in the case of kittens. It is advisable that a rat only be shown twice while a kitten. Kittens should not be taken to exhibitions where it's expected that they would be handled by the general public.
During hot weather adult bucks should be left at home as they are very prone to heat stroke.