To determine the relative roles of the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei in the control of the release of vasopressin and of ACTH, we have examined the hormonal responses to electrical stimulation (200 microA, 0.2 msec, 100 Hz, 20 sec) of these regions. Cats were anesthetized with chloralose-urethane. Blood samples were taken 30 sec before stimulation and 1.5 min poststimulation. ACTH and vasopressin were measured by RIA. Electrical stimulation of the caudal pole of the SON increased vasopressin in plasma (1.82 +/- 0.41 microU/ml, n = 17, P less than 0.01) and decreased ACTH (-26 +/- 4 pg/ml, n = 13, P less than 0.01). In contrast, stimulation of the PVN increased vasopressin (2.01 +/- 0.60 microU/ml, n = 7, P less than 0.001) and increased ACTH (107 +/- 20 pg/ml, n = 32, P less than 0.01). Previous work has shown that vasopressinergic neurons of PVN, but not of SON, project to the zona externa of the median eminence. Other have suggested that the retrograde flow of blood from the neural lobe to the median eminence and thence to the anterior lobe would allow vasopressin to influence the release of ACTH. The present results indicate that both SON and PVN facilitate the release of vasopressin. However, PVN facilitates, but SON inhibits the release of ACTH. These findings suggest that the projection from PVN to the zona externa of the median eminence mediates the release of ACTH and that retrograde flow from the neural lobe is not important in the control of ACTH release during modest and transient increases in the release of vasopressin.