Combinations of morphine and d-amphetamine have been shown to be more effective in relieving pathological and experimental pain with fewer side effects than morphine alone. Subjective, physiological and behavioral effects of the combination were assessed in the present study to determine whether the abuse liability of the combination might also be enhanced. Single intramuscular doses of morphine and d-amphetamine alone and in combination were tested under double-blind conditions in 10 adult, male, non-dependent substance abusers. Morphine and amphetamine alone increased scores on subjective effect measures including the Morphine-Benzedrine Group (MBG) (euphoria) and liking scales. In general, the combination produced greater euphoria than that produced by either drug alone. In contrast, many physiological effects of the drugs were mutually antagonized. Thus, the combination of morphine and d-amphetamine has a greater potential to be abused because of the additive euphoria and a lessening of side effects.