Previous studies have shown that activation of the 5-HT1A 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-HT1A agonist, into the PVN increased rat plasma ACTH concentration in a dose-related manner. Similar responses were observed when two other 5-HT1A agonists, buspirone and gepirone, were used. (± )-Pindolol, known to have 5-HT1A 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-HT1A receptor subtype activation is mediated by the release of CRH from the paraventricular nuclei of the hypothalamus in rats, but do not exclude other mechanism