These studies were designed to explore a peculiar behavior displayed by rats during the acquisition of heroin self-administration (0.05 mg/kg/infusion) on a fixed-ratio 1 schedule of reinforcement in limited access conditions (i.e. 3 h/day). Rats trained under these conditions develop a tendency to emit extra lever presses during the time of heroin infusions (unreinforced responses). We found that a similar behavior develops in animals responding for sucrose pellets, but not for intravenous infusions of cocaine (0.5 mg/kg/infusion, 3 h/day). In sucrose trained rats, unreinforced responses emitted during the delivery of sucrose pellets was enhanced by food deprivation. In heroin trained rats, development of unreinforced responding was accompanied by an increase in responding for heroin on a progressive ratio schedule, and by a reduction of the depressant action of heroin (3 mg/kg, SC) on locomotor activity. On the basis of these findings, we concluded that unreinforced responding during heroin self-administration reflects a change in the motivation to obtain the drug, as well as a reduced sensitivity the motor impairing action of heroin. This suggests that acquisition of heroin self-administration is regulated by a balance between drug effects that promote and limit heroin intake.