Data from previously reported studies suggest that methadone may be more euphorigenic than other morphine-like drugs. To determine whether methadone is more potent in producing euphoria relative to its other opiate-like effects, single doses of intravenously administered methadone, morphine, heroin and placebo were compared in non-dependent post-addict volunteers. Morphine-like physiologic, subjective and behavioral effects were measured periodically for 24 h after drug administration. Under the conditions of the experiment methadone produced a profile of effects which was indistinguishable from that of morphine and heroin. Only the time course of miosis, which was longer lasting following methadone, differentiated among the three compounds. The relative potencies of methadone, morphine and heroin for the initial 5 h of effect were constant over all opiate-like effects, including measures of euphoria. Thus, methadone was not a selective euphoriant.