Previous research demonstrates that repeated intravenous (i.v.) nicotine injection resulted in increased locomotor sensitization in female relative to male rats. In order to determine if increased nicotine levels are detected in females compared to males the present experiment examined the plasma nicotine levels of male, female, castrated (CAST), and ovariectomized (OVX) rats (n=7-11 rats/group) following repeated i.v. nicotine injection (50 microg/kg/injection). All rats received 14 i.v. nicotine injections, one/day. Approximately 1 min after the 14th nicotine injection, rats were rapidly decapitated and trunk blood was collected immediately. Gas chromatography revealed a sex difference in nicotine content: higher plasma nicotine levels were measured from female rats (>10 x increase) relative to males, and the sex difference was attenuated by gonadectomy. These data suggest that the sex difference in plasma nicotine levels is due to alteration in distribution or nicotine metabolism as a function of circulating gonadal hormones. These findings indicate that gonadal hormones may influence nicotine pharmacokinetics and therefore nicotine-induced sex differences in behavior.