The respiratory depressant and analgesic effects of intravenous dezocine were evaluated in six healthy volunteers. Single 0.15 mg/kg doses were compared with identical amounts of morphine, and the two drugs were given in combination. Five successive 0.15 mg/kg doses of dezocine also were given to identify dose-effect relationships. Respiratory center sensitivity was monitored by carbon dioxide (CO2) rebreathing and mouth occlusion pressure (P0.1) measurements, while analgesia to experimental pain was tested with submaximal tourniquet ischemia. Single 0.15 mg/kg doses of dezocine produced significantly more tolerance to experimental pain and greater respiratory depression than a comparable dose of morphine in the first hour, but effects of both drugs were similar thereafter. Multiple doses of dezocine progressively increased pain tolerance from 46 ± 14% above control with the first dose to 70 ± 18% above control with the second dose (cumulative total 0.30 mg/kg). Additional dezocine doses did not result in significantly more analgesia. Depression of CO2sensitivity followed a similar pattern. Morphine 0.15 mg/kg, when given to subjects who had received a prior dose of dezocine, produced no additional effect beyond that observed with dezocine. With the reverse sequence, dezocine increased the respiratory depression of morphine but also produced a dramatic increment in analgesia, which suggested an additive action. Dezocine is therefore an effective analgesic with morphine-like effects. In human subjects it appears to be a slightly more potent analgesic than morphine in identical clinical doses (0.15 mg/kg). Dezocine is similar to other agonist-antagonist analgesics in that it exhibits a ceiling effect for respiratory depression that parallels its analgesic activity.