The purpose of this study was to present a comparative and detailed picture of behavioral, physiological, and motor effects of some barbiturates, benzodiazepines, and neuroleptics, using an operant procedure of differential reinforcement of response duration (DRRD) in the dog. An increase in response rate with low doses of barbiturates and benzodiazepines is interpreted in terms of behavioral disinhibition, anxiolytic effect, and enhancement of motivation. The depressive effects noted with higher doses of benzodiazepines and barbiturates, as well as with all doses of neuroleptics, can be explained in different ways. For benzodiazepines and barbiturates, ataxic effects more than sedative or motivational effects are implicated. However, for neuroleptics, loss of motivation or "anhedonia" better explains the decrease in operant responding. Differences between the main classes of psychotropic drugs but also within a class between 1,4 benzodiazepines or 1,5 benzodiazepines, hypnotic or nonhypnotic benzodiazepines, and typical or atypical neuroleptics are highlighted and correlated with human observations.