The effects of etorphine, a potent morphine-like drug, were qualitatively and quantitatively compared to those of morphine. In nondependent subjects, etorphine in doses of 0.025, a .050, and 0.100 mg produced pupillary constriction and morphine-like subjective effects and euphoria. Etorphine was 500 times as potent as morphine, with a very rapid onset and short duration of action. In morphine-dependent subjects, etorphine suppressed abstinence but for a shorter period than morphine. These studies indicate that in man etorphine is a morphine-like drug with a high abuse potential.