In a double-blind, single-dose study, dextroamphetamine combined with morphine was compared with morphine alone to determine the relative efficacy of the combination given intramuscularly for postoperative pain. Each of 450 patients received one treatment of morphine sulfate (3, 6 or 12 mg) with dextroamphetamine (0, 5 or 10 mg). Analgesia, as measured by the patients' subjective responses to questions about relief of pain, was augmented when dextroamphetamine was given with morphine; the combination of dextroamphetamine, 10 mg, with morphine was twice as potent as morphine alone, and the combination with 5 mg was 1 1/2 times as potent as morphine. In simple performance tests, and in measures of side effects, dextroamphetamine generally offset undesirable effects of morphine (sedation and loss of alertness) while increasing analgesia. Effects on blood pressure, pulse and respiratory rate were minimal.