The alpha 1-adrenergic receptor blocking effect of ketanserin, the blocking properties of this compound for nonspecific stimulation with angiotensin II, and the alpha-adrenergic receptor blocking properties and the blood pressure lowering effect of phentolamine, ritanserin, and the combination of both compounds were studied in patients on cardiopulmonary bypass (constant flow rate, mild hypothermia) undergoing coronary artery bypass grafting. Phenylephrine was used as alpha 1-adrenergic agonist. Ketanserin reduces the alpha 1-agonistic effect of phenylephrine on blood pressure in a dose-dependent manner up to a dose of 10 mg. Ketanserin did not block the nonspecific vasoconstriction, as induced by angiotensin II. The moderate blood pressure lowering effect of phentolamine was substantially potentiated by ritanserin, which in itself did not affect blood pressure. The findings in this study indicate that the blood pressure lowering activity of ketanserin results from a combined blockade of alpha 1-adrenergic and S2-serotonergic receptors.