Studies on the role of endogenous sodium pump inhibitors in hypertension have used ouabain, a Na,K-ATPase inhibitor, as a pharmacological probe to test the effect of direct vascular Na,K-ATPase inhibition in man. Previous work has suggested that there are no alpha-adrenoceptor mediated components in the vasoconstrictor effect of this drug, although several animal and in vitro studies have shown a complex interaction between ouabain and either sympathetic neurotransmission or alpha-adrenergic excitation-contraction coupling mechanisms. To re-examine this issue, we infused ouabain at 7 micrograms/dl per min for 20 min without (saline infusion) or with phentolamine (8.0 micrograms/dl per min for 15 min), an alpha-adrenoceptor blocking agent, in uncomplicated mild to moderate hypertensive patients. The drugs were infused into the brachial artery, and forearm blood flow (by strain-gauge plethysmography), systemic blood pressure and heart rate were measured concomitantly. After local phentolamine pretreatment, ouabain lost its vasoconstrictor effect (n = 5). This result is unlikely to have been related to vasodilation because histamine, a non-alpha-adrenergic vasodilator, infused in another group (n = 5) of patients at rates roughly equieffective to phentolamine (0.05 microgram/dl per min for 15 min) did not modify the vasoconstrictor action of ouabain. Thus alpha-adrenoceptor mediated components caused the forearm vascular effect of ouabain and must be taken into account when using ouabain to study endogenous sodium pump inhibitor(s) in human hypertension.