The motivational effects of exclusively peripheral or central opiate receptor blockade were studied using place conditioning. Place aversions were observed with intraventricular (i.c.v.) methylnaloxone (MN) in both naive (200-1000 ng) and morphine-dependent rats (50-500 ng). Subcutaneous MN (0.03-10 mg/kg) was ineffective in naive rats; in dependent rats a small aversion was seen at the highest dose. Place aversions were not necessarily associated with behavioral signs of withdrawal. The data suggest that the aversive properties of opioid receptor antagonism are centrally mediated in both naive and dependent rats, and that their enhancement in morphine-dependent subjects results from a sensitized central mechanism rather than from the recruitment of a peripheral component.