Cocaine induces an increase in hippocampal and nucleus accumbens (Nac) serotonin (5-HT) concentration parallel to locomotor activation. Both effects can be modulated by systemic 5-HT 1A -receptor agonism/antagonism. Given the contribution of the hippocampus to spontaneous behavioral activity, these observations suggest a role for hippocampal 5-HT as well in the modulation of cocaine effects on behavior. To determine the role of hippocampal 5-HT 1A -receptors in cocaine effects on behavior and hippocampal 5-HT release, we used in vivo microdialysis in freely moving rats. The 5-HT 1A -receptor agonist, 8-OH-DPAT (0, 0.1, 1 and 10 M), was applied locally into the hippocampus by reversed dialysis followed by a cocaine (10 mg/kg) or saline i.p. injection. The hippocampal 5-HT 1A -receptor activation attenuated cocaine-induced hyperlocomotion and rearing behavior dose-dependently. Parallel to that, the cocaine-induced 5-HT increase was attenuated dose-dependently in the hippocampus but was left unaffected in the Nac. The intra-hippocampal application of 8-OH-DPAT affected neither behavioral activity nor 5-HT concentration in the hippocampus and in the Nac. In accord with these findings, hippocampal 5-HT 1A -receptors may not be directly involved in the regulation of spontaneous behavior or basal 5-HT concentration in the hippocampus and Nac. However, the results indicate an inhibitory role of hippocampal 5-HT 1A -receptors in cocaine-induced hyperactivity and in the 5-HT increase evoked by cocaine in the hippocampus but not in the Nac.