• In the context of evaluating the effects of a narcotic antagonist on opiate acquisition, 14 detoxified addicts self-administered increasing doses of unblocked heroin intravenously over a ten-day period. Early in the addiction cycle, subjects experienced tension relief and euphoria but this was followed shortly by a shift in the direction of increasing dysphoria and psychopathology. Nonetheless, individual injections of the drug continued to induce brief episodes of positive mood, an effect enhanced by frequent injection. Heroin self-administration was sharply reduced when subjects were blocked with naltrexone, a narcotic antagonist, and the negative effects observed during unblocked drug use were not observed.