The selective 5-HT1A receptor ligand ipsapirone (IPS) induces hypothermia in humans. To explore 5-HT1A receptor-mediated thermoregulation in depression, 24 subjects (12 patients with unipolar depression and 12 individually matched controls) received 0.3 mg/kg IPS or placebo in random order. Compared with controls, the depressed patients exhibited significantly attenuated hypothermic responses to IPS. The impaired hypothermic response following 5-HT1A receptor activation in unipolar depression could have resulted from subsensitivity of the (presynaptic) 5-HT1A receptor and/or related effector mechanisms, thus supporting the hypothesis that altered serotonergic activity may be present in affective disorders. Future studies of the hypothermic response to direct-acting 5-HT1A ligands, such as IPS should facilitate the assessment of 5-HT receptor function in various affective disorders and its involvement in psychotropic drug effects.