The serotonin (5-HT) agonist 1-(m-chlorophenyl)piperazine (mCPP) has been widely used as a pharmacological probe to assess 5-HT function. Although mCPP is known to interact with 5-HT receptors, this drug is also reported to exhibit presynaptic actions that increase extraneuronal 5-HT in vitro. In the present study, we used in vivo microdialysis to examine the effects of mCPP on extracellular 5-HT in the ventromedial diencephalon of awake rats. Intravenous mCPP (1.0 and 2.0 mg/kg) increased dialysate 5-HT in a dose-related manner, with extracellular 5-HT levels rising 8-fold above baseline after the high dose of drug. The stimulatory effect of mCPP on dialysate 5-HT was abolished by pretreatment with the 5-HT uptake blocker fluoxetine (10 mg/kg, i.p.). In complementary experiments, mCPP elevated plasma prolactin at doses equivalent to those that increased dialysate 5-HT, and fluoxetine pretreatment caused a partial, though significant, attenuation of mCPP-induced prolactin release. These results indicate that mCPP increases extracellular 5-HT in rat brain by a presynaptic mechanism involving 5-HT transporters. Moreover, the plasma prolactin response to mCPP is at least partially mediated by the presynaptic actions of the drug. Our data further suggest the possibility that mCPP exhibits indirect agonist properties in human brain. Therefore, clinical studies designed to evaluate postsynaptic 5-HT receptor sensitivity based on responsiveness to mCPP should be interpreted with caution.