This study explored the relation between the reinforcing properties of different drugs and their subjective effects. Preference for a drug (amphetamine or diazepam) over placebo was measured in a choice procedure, and mood changes after drug administration were monitored by self-report questionnaires. Individual differences in behavioral drug preferences were then examined for their relationships to subjective drug responses as well as other subject variables. In Part I, the subjective effects of amphetamine were compared in those subjects who consistently preferred (i.e. chose) amphetamine over placebo, and those who preferred placebo over amphetamine. The two subject groups showed markedly different subjective responses to the stimulant drug: the choosers reported increased positive mood and euphoria, whereas the non-choosers reported only increased anxiety and depression. In Part II, the subjective effects of diazepam in consistent choosers of diazepam were compared to the subjective effects in consistent non-choosers of the drug. The non-choosers showed appreciable subjective effects, which were predominantly sedative in quality. In contrast to the results with amphetamine, the choosers showed negligible subjective drug effects. These results underline the importance of considering more than one response measure when attempting to characterize the reinforcing effects of drugs.