Intravenous doses of butorphanol tartrate (0.5 mg, 1.0 mg and 2.0 mg) and meperidine hydrochloride (20 mg and 40 mg) were compared under controlled conditions employing a double blind study design. Informed consent was obtained from all post-operative patients suffering from moderate to severe pain who participated in this study. Approximately 25 patients were included in each group. The data from 125 patients were subjected to statistical analysis. The results indicated that butorphanol is approximately 40 to 50 times more potent than meperidine. In addition, at most of the time intervals, there were no statistically significant differences between the responses to butorphanol 0.5 mg and 1 mg and meperidine 20 mg and 40 mg; but the response to butorphanol 2 mg was significantly (p less than 0.05) better than the low dose of each agent. The low doses of butorphanol (0.5 mg) and meperidine (20 mg) appear to have an effective duration of action of less than two hours. The larger doses (butorphanol 1.0 mg and 2.0 mg and meperidine 40 mg) appeared to produce a two- to four-hour duration of action. The largest butorphanol dose (2.0 mg) appeared to produce the longest duration of action. A comparison of the test groups with respect to the incidence and type of side effects showed that butorphanol 2.0 mg produced a greater incidence of drowsiness (39 per cent). The overall incidence of drowsiness for patients receiving either the 0.5 mg or 1.0 mg dose of butorphanol was 12 per cent, as compared with an 8 per cent overall incidence in the meperidine group. The incidence of other side effects was relatively low in all test groups. No significant differences were noted among the groups with regard to the onset (usually less than or equal to 30 minutes post-therapy) or the duration (usually less than or equal to 2 hours) of side effects. Butorphanol appears to be a safe and effective analgesic for the relief of moderate to severe post-operative pain.