Although the role of genetic factors in the response to drugs of abuse has been emphasized, no earlier studies have applied selective breeding to intravenous drug self-administration. Here we report the effects of six generations of selective breeding for rat lines with low or high levels of intravenous drug self-administration (LS and HS lines, respectively). Rats from the outbred founder population and the first selected generation were evaluated for intravenous self-administration of either morphine or cocaine. All subsequent generations were assessed for self-administration of cocaine, using a multifactorial score based on how rapidly self-administration behavior was acquired, levels of self-administration during acquisition, and the response to different doses of cocaine. All changes in cocaine self-administration that occurred in generations three through six were consistent with effects of selection, with most measures differing in sixth-generation LS and HS animals. Sixth-generation HS rats self-administered approximately five times more injections of low-dose cocaine than LS animals under fixed-ratio-5 (a schedule in which an injection is delivered after five lever presses). These findings support a role of genetic factors in influencing cocaine-reinforced behavior. Establishment of the LS and HS lines will allow future studies to evaluate the role of specific genetic factors that underlie these differences and may contribute to substance abuse disorders in humans.