A project has been initiated to breed lines of rats selected for high and low degrees of tolerance to ethanol. At the limit of selection, animals selected for each trait will theoretically contain all alleles associated with that trait, while alleles not associated with the trait will be randomly distributed. Therefore, these animals will be a resource for all researchers interested in the biochemical determinants of alcohol tolerance. Animals will inhale ethanol chronically in specially-constructed chambers, and will then be tested for functional tolerance to the hypnotic effect of ethanol. Animals demonstrating high or low degrees of tolerance will be mated, with precautions taken to minimize inbreeding.