Previous studies have shown that activation of the 5-HT<sub>1A</sub> receptor subtype enhances rat plasma ACTH concentration. Such receptors have been suggested to be located on CRH neuronal cell bodies in the paraventricular nuclei of the hypothalamus (PVN). In this report, microinjection of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a selective 5-HT<sub>1A</sub> agonist, into the PVN increased rat plasma ACTH concentration in a dose-related manner. Similar responses were observed when two other 5-HT<sub>1A</sub> agonists, buspirone and gepirone, were used. (± )-Pindolol, known to have 5-HT<sub>1A</sub> antagonist properties, blocked the effect induced by an optimal dose of 8-OH-DPAT after injection into the PVN. This same dose of 8-OH-DPAT also induced a decrease of hypothalamic CRH concentration, which was completely antagonized as well by pretreatment injection of ( ± )-pindolol into the PVN. A significant inverse correlation was found between hypothalamic CRH and plasma ACTH levels. These results confirm that elevation of the plasma ACTH concentration induced by 5-HT<sub>1A</sub> receptor subtype activation is mediated by the release of CRH from the paraventricular nuclei of the hypothalamus in rats, but do not exclude other mechanisms.