The long-term hemodynamic effects of atenolol were studied in 10 patients with previously untreated essential hypertension in WHO stage 1. Oxygen consumption, heart rate, cardiac output, and intra-arterial brachial pressure were recorded at rest in supine and sitting positions and during steady-state work at 50, 100, and 150 W. The patients were treated with 100 mg atenolol daily (200 mg in 1 patient) as the sole antihypertensive drug and were restudied after 1 and 5 years. After I year the blood pressure was reduced approximately 18% both at rest and during exercise, and the heart rate about 25%. The cardiac output was reduced 16% at rest supine. 27% at rest sitting, and about 20% during exercise. The calculated total peripheral resistance was not decreased compared to pretreatment values. After 5 years on treatment, the hemodynamic parameters were almost identical to those seen after I year. There was no increase in stroke volume or cardiac output and no decrease in the total peripheral resistance. Thus prolonged beta-blocker treatment over several years does not seem to normalize hemodynamics in patients with mild to moderate essential hypertension.