Rats hate the smell of all types of mint, cat urine, moth balls and ammonia. Additionally, rats are repelled by food waste that's fermented using Bokashi, a Japanese method for breaking down organic matter before it's composted.
Rats avoid the smell of cat urine because they equate it with death. Mint and citronella plants repel rats when grown around gardens, and mint's essential oils work to deter rats as well. Napthalene balls repel rats with their strong odor, as does ammonia, which is typically diluted with distilled water and applied to the carpets or left in open containers in the areas rats frequent to deter them.