Serotonin (5-HT) is known to be a potent stimulator of aldosterone release in vitro and, to a lesser degree, in vivo. Since ketanserin, a specific 5-HT2-receptor antagonist, decreases aldosterone levels in some hypertensive patients, we have performed in vitro experiments to elucidate the mechanism by which ketanserin interferes with aldosterone regulation. After collagenase dispersion, rat zona glomerulosa cells were either perfused and resuspended in Biogel or incubated in Earle's medium and bovine serum albumin. The cells were stimulated with serotonin, angiotensin II, potassium (K+) and adrenocorticotrophic hormone (ACTH) in the presence or absence of ketanserin. Ketanserin did not decrease baseline aldosterone secretion although it reduced the stimulatory capacity of serotonin and K+, and had a minimal effect on ACTH. Furthermore, we demonstrated that ketanserin is able to have a significant effect on the adrenal response to angiotensin II. These data indicate that antiserotoninergic agents may act directly at the level of the adrenal glomerulosa by interfering with specific serotoninergic receptors and modifying the adrenal sensitivity to angiotensin II and K+. The importance of these findings in the clinical use of these antihypertensive drugs remains to be established.