On a residential research ward, the acute effects of placebo, 0.5 to 3.0 mg of triazolam (TZ) and 100 to 600 mg of pentobarbital (PTB) were examined using a within-subject, double-blind design in male volunteers with documented histories of drug abuse. Drug effects were examined through the use of subject ratings including measures of drug liking and estimates of street value, staff ratings, objective psychomotor/cognitive performance measures, subject estimates of performance, immediate and delayed recognition memory tasks and subject ratings of nighttime sleep quality. Staff ratings and objective performance measures showed that TZ and PTB produced comparable dose-related impairment; TZ had a more rapid onset and a shorter duration of action than PTB. With these measures, TZ was 159 to 274 times more potent than PTB. With subject-rated measures of drug effect, sleepiness and drunkenness, in contrast, TZ produced smaller effects than PTB or was only 135 to 163 times more potent than PTB. Similarly, with subject ratings of drug liking and estimated street value, TZ produced smaller effects than PTB and was only 91 to 122 times more potent than PTB. Other results showed that TZ produced greater amnestic effects than PTB and subjects under the influence of TZ more consistently underestimated the degree of their impairment. Overall, these results suggest that TZ has a lower liability for abuse (likelihood) than PTB, but a greater liability of abuse (hazard) with regard to performance impairment on certain kinds of tasks.