Lever-pressing behavior reinforced by intravenous infusion of various concentrations of heroin, and consequent development of physical dependence, were examined in rats. In addition, the influence of opiate dependence, and of its disappearance following withdrawal, on heroin self-administration were investigated. It was found that intravenous self-administration of heroin at 0.03 mg/kg/infusion maintained self-administration behavior without producing physical dependence. Total responses per session decreased with increasing unit dose of heroin, whereas the total amount of drug self-administered was directly related to unit dose. Significantly greater numbers of withdrawal signs and percentage body weight losses in response to naloxone injections were observed following self-administration of heroin at 0.1, 0.3 or 0.6 mg/kg/infusion. Intake of heroin at 0.03 mg/kg/infusion, but not at 0.1, 0.3 or 0.6 mg/kg/infusion, was found to increase significantly in opiate-dependent and postdependent animals. These findings support the previous use of 0.03 mg/kg/infusion as a suitable dose for illustrating the reinforcing effect of heroin without the influence of physical dependence.