In vivo microdialysis in guinea pig hypothalamus was used to study the effect of serotonin [5-hydroxytryptamine (5-HT)] subtype 1D autoreceptor blockade on the increase in extracellular 5-HT levels produced by a selective 5-HT reuptake inhibitor (SSRI). Administration of the selective 5-HT1D antagonist GR127935 at 0.3 mg/kg had no effect, but 5 mg/kg significantly increased extracellular levels of 5-HT and 5-hydroxyindoleacetic acid to 135% of basal values. Moreover, at these doses GR127935 significantly attenuated the decrease in extracellular 5-HT levels following local perfusion with the selective 5-HT1D agonist CP-135,807. The SSRI sertraline at 2 mg/kg increased 5-HT levels to 130% of basal levels. The combination of this low dose of sertraline with either dose of GR127935 resulted in a pronounced, long-lasting increases in 5-HT levels to 230% of basal values. These results indicate that the effects of an SSRI on terminal 5-HT are significantly enhanced by coadministration of a 5-HT1D antagonist and confirm that in addition to somatodendritic 5-HT1A autoreceptors, terminal 5-HT1D autoreceptors mitigate the effect of SSRIs on terminal 5-HT. As such, antagonists of the 5-HT1D autoreceptor could be useful as rapidly acting antidepressants and may shorten the onset of antidepressant action when combined with SSRIs.