This study investigated how different stages of cocaine self-administration in rats affect the expression of two serotonin receptors in dorsal and ventral striatum, the 5-HT(1B) and 5-HT(6) subtypes, which have both been implicated in mediating some aspects of cocaine-related behaviors. In the first experiment, rats were trained to work for saccharin (oral) or cocaine (i.v.) reinforcers. We found that continuous access to cocaine for 23 days did not change the level of 5-HT(1B) mRNA expression compared to control animals receiving saccharin. However, a single cocaine session, given either by self-administration or non-contingently, increased 5-HT(1B) mRNA in dorsal striatum, whereas forced abstinence for two weeks after cocaine reduced 5-HT(1B) mRNA expression in the same subregion. 5-HT(6) mRNA was not changed by any of these treatments. A follow-up experiment investigated the effects of limited versus extended access to cocaine as well as forced abstinence, and we found that 14 days of forced abstinence significantly reduced 5-HT(1B) mRNA throughout the dorsal and ventral striatum compared to no withdrawal. These results suggest that the influence of 5-HT(1B) receptors in striatal projection neurons may be increased during cocaine acquisition and reduced after forced abstinence and may therefore be targets for pharmacological intervention in addiction.