Extracellular levels of serotonin (5-HT) and the regulation of 5-HT release by the 5-HT1A receptor were examined after single and repeated treatment with different types of antidepressant drugs: the selective 5-HT uptake inhibitor fluoxetine, the selective norepinephrine uptake inhibitor desipramine and the 5-HT2A/2C/alpha 2 receptor antagonist mianserin (each at 15.0 mg/kg). Extracellular levels of 5-HT were measured using in vivo microdialysis in the striatum and hippocampus of rats anesthetized with chloral hydrate. Acute administration of fluoxetine transiently elevated the levels of 5-HT in the striatum and hippocampus; desipramine did not change 5-HT levels, and mianserin slightly decreased 5-HT levels in the hippocampus. Rats were administered these antidepressant drugs for either 1 or 14 days and studied 48 hr after the final injection. Repeated treatment with fluoxetine increased base-line levels of 5-HT in the striatum and hippocampus; repeated treatment with desipramine increased base-line 5-HT levels in the striatum only, and repeated treatment with mianserin did not alter base-line 5-HT levels. Repeated fluoxetine treatment attenuated the ability of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) to decrease 5-HT release in both the striatum and hippocampus. Repeated desipramine treatment did not significantly alter the effects of 8-OH-DPAT on 5-HT release, but there was a hint of a decreased effect in the hippocampus. Repeated mianserin treatment did not significantly alter the effects of 8-OH-DPAT on 5-HT release, but there was a hint of an increased effect in the striatum. The results of the present study suggest that repeated treatment with antidepressant drugs alters extracellular levels of 5-HT and the ability of 5-HT1A receptors to regulate the release of 5-HT in a regionally selective manner. These changes in the regulation of 5-HT release produced by antidepressant drugs may be associated with their therapeutic effects, because they are caused by repeated rather than acute treatment.