Behavioural responses to psychostimulant drugs can be profoundly affected by early environmental influences. The aim of this study was to describe the effects of repeated brief separations of rat pups from their dams during the early neonatal period on cocaine self-administration behaviour as adults. Lister hooded rats exposed to a repeated maternal separation procedure (REMS) showed altered acquisition and maintenance of cocaine self-administration as adults, the effects being dose and gender-dependent. Overall, the patterns of acquisition of self-administration across three doses of cocaine (0.05, 0.08 and 0.5 mg/injection) suggested a rightward shift in the acquisition dose-effect functions for the REMS animals relative to control animals. At 0.05 mg/injection, there was a retarded acquisition of cocaine self-administration in male and female neonatally separated rats. At 0.08 mg/injection there was a facilitated acquisition in female neonatally separated subjects. After establishment of stable self-administration of the training dose, in the same cohort of subjects, rightward and downward shifts in the cocaine self-administration dose-effect functions were determined for female and male REMS subjects, respectively, relative to their controls. The dose-effect function for both female groups was shifted to the left of that of the respective male groups, although the lighter body weights of the females meant that they administered a higher unit dose per unit body weight than the males. Whereas male REMS subjects tended to self-administer less cocaine than the controls at the dose eliciting maximal responding (0.03 mg/injection) and to make fewer lever responses overall at each dose tested, female REMS subjects self-administered significantly more cocaine than their respective controls at a dose of 0.03 mg/injection. There was no differential sensitivity to the rate-altering effects of the selective dopamine D2 receptor antagonist, eticlopride, or to the selective dopamine D1 receptor antagonist, SCH 23390. These data provide further evidence that altered early environment affects drug-taking behaviour in a developmentally specific and gender-specific manner, with the effects of neonatal separation contrasting with previously published data on the effects of post-weaning isolation rearing.