The effects of diazepam (10-160 mg) and oxazepam (30-480 mg) were studied in volunteers with histories of drug abuse. Oral doses were administered every third day under double-blind and counterbalanced conditions. Dose-effects with area under the time-action curve data (AUC) showed diazepam to be 2.6-5.7-times more potent than oxazepam on various psychomotor, cognitive, staff-rated, and subjective measures. Comparison of relative potencies showed diazepam to be relatively more potent in producing 'liking' than in producing psychomotor and cognitive effects. Diazepam produced greater peak effects than oxazepam on a number of staff- and subject-rated measures, including liking. Onset of effect was more rapid and time to maximal effect was shorter (1-2 h versus 4-12 h) with diazepam than oxazepam, while time to offset of effect was similar for the two drugs. Diazepam was categorized as producing barbiturate-like subjective effects (38.3%) more frequently than was oxazepam (13.8%), while oxazepam was identified as placebo more often than diazepam. Repeated administration of 160 mg diazepam and 480 mg oxazepam showed that AUC liking was greater for diazepam than oxazepam and that tolerance to psychomotor and cognitive effects occurred with oxazepam but not diazepam. This study suggests that diazepam may have a higher abuse liability than oxazepam.