Drug discrimination is a method of behavioral pharmacology which establishes drugs as discriminative stimuli to behavioral responses. The drug discrimination paradigm typically involves the training of animals to discriminate the administration of a given chemical from that of its vehicle. Prominent features of the paradigm are its exquisite pharmacological specificity, reliability and sensitivity. Toxicology can utilize drug discrimination in at least two ways. Firstly, discriminative effects often signal the presence of chemicals to exposed subjects. Secondly, drug discrimination offers a standardized and well controlled preparation in which measurements are available that are sensitive to possible defects in numerous neurobiological and behavioral functions. Drug discrimination is thus sensitive to defects in neurotransmitter systems, drug-receptor interactions, learning, memory, arousal, motor control, drives, etc. Drug discrimination can be viewed as an encompassing and sensitive technique for the advanced toxicological evaluation of drugs and environmental chemicals.