Acute stimulation of the hypothalamo-pituitary-adrenal (HPA) axis by selective serotonin reuptake inhibitors (SSRIs) is mediated by several postsynaptic 5-HT receptor subtypes. Activation of 5-HT(1A) and 5-HT(2A) receptors increases plasma corticosterone levels, and it is likely that these receptor subtypes are central to mediating the effects of SSRIs. To study the interaction of these receptors, rats were administered with the 5-HT(1A/7) agonist 8-OH-DPAT (0.05 to 1.25 mg/kg), the 5-HT(2A/C) agonist DOI (0.25 to 5 mg/kg), or a mixture of both compounds, and trunk blood was taken 60 min later. The two compounds given in combination produced a lower increase in corticosterone than DOI does alone. DOI and 8-OH-DPAT also produced a marked induction of c-Fos in the paraventricular nucleus (PVN), but the induction was not different if the two compounds were given together. These data show that the two serotonin receptor subtypes affect the HPA axis via a central target. In conclusion, 5-HT(1A) and 5-HT(2A) receptors regulate corticotrophin-releasing hormone (CRH) neurons via distinct but strongly interacting pathways, probably converging on the same neurons in the hypothalamus.