In 6 patients with untreated hypertension of mild or moderate degree, dilevalol was infused in the brachial artery. Doses were calculated to produce plasma levels approximating those achieved after oral dosing (0.03, 0.07, 0.1, 0.3 micrograms.kg-1.min-1) and also to produce plasma levels exceeding oral dosing (0.5, 1.0, 2.0 micrograms.kg-1.min-1) without causing any blood pressure or heart rate changes. The effects of the infusion on forearm blood flow were assessed by venous occlusion plethysmography. Dilevalol caused a progressive increase in flow in all subjects up to the intermediate dose given, with the effect being attenuated when the dose was increased further. Simultaneous propranolol infusion reduced the increase in flow induced by dilevalol, shifting the dilevalol-induced vasodilation dose response curve to the right. Isoproterenol infusion caused a marked, dose-related increase in flow that was equally well reduced by simultaneous infusion of dilevalol or propranolol. These results indicate that dilevalol effectively blocks peripheral vascular beta receptors in humans. The drug also acts as a partial beta 2-receptor agonist, causing vasodilation which can be reduced by co-administration of propranolol.