In eight patients with essential hypertension the effect of 50 mg atenolol, once daily for 6 months, on vasodilation during epinephrine infusion and submaximal dynamic exercise was studied. The normal decrease of diastolic blood pressure during bicycle exercise, reflecting a decrease in total peripheral resistance not mediated by circulating epinephrine, disappeared during atenolol treatment. Low-dose infusion of epinephrine had no influence on systolic blood pressure both before and after atenolol. However, the decrease of diastolic blood pressure occurring before atenolol was abolished and the increase in heart rate was attenuated during atenolol treatment. Forearm vascular resistance decreased before and during atenolol to the same extent. So the normal physiologic vasodilation during submaximal dynamic exercise seems impaired during long-term treatment with atenolol. In addition the normal vasodilating response to an increase of circulating epinephrine to levels occurring during daily life stress seems impaired even with the low dose of this beta 1-selective beta-blocker.