The effects of dopamine (DA) and 5-hydroxytryptamine (5-HT) autoreceptor agents on electrically induced [3H]DA and [3H]5-HT release from superfused slices of striatum, nucleus accumbens and ventral mesencephalon (VM) containing A9 and A10 neurons were investigated in rats made tolerant to the stimulatory effect of cocaine on locomotor behavior by a 14-day continuous infusion of cocaine (29 mg/kg/day) by s.c. implanted osmotic minipumps followed by a 7-day drug-free period. In VM, electrically induced [3H]DA was increased, the ability of pergolide to inhibit this release was abolished, but the ability of sulpiride to facilitate the release was potentiated, implicating a higher concentration of synaptic DA modifying the responsiveness of somatodendritic D2 autoreceptors to D2 agents. Both electrically induced [3H]5-HT release from VM and the stimulatory effect of in vitro cocaine on this release were enhanced whereas the effects of both 5-methoxytryptamine and methiothepin were attenuated, indicating that subsensitivity of 5-HT autoreceptors developed in DA cell body regions. In striatum and nucleus accumbens, no significant changes were observed in [3H]DA and [3H]5-HT release, except for a modest reduction in the effects of both pergolide and sulpiride on electrically induced [3H]DA release from striatum. These results emphasize the importance of pretreatment-induced changes in DA cell body regions, rather than terminal areas, under the present conditions. The observed increase in DA autoinhibitory tone and subsensitivity of 5-HT release-regulating autoreceptors in the VM may contribute to the locomotor tolerance upon cocaine challenge after continuous cocaine.