1 Thirteen men with untreated essential hypertension in WHO stage I were studied on an outpatient basis to evaluate the haemodynamic long-tern effect of a new beta-adrenoceptor blocker, atenolol. 2 Oxygen consumption, heart rate, cardiac output (Cardiogreen) and intraarterial branchial pressure were recorded at rest in a supine and sitting position and during steady state work at 300, 600 and 900 kpm/min. 3 The subjects were treated with atenolol (dose 100-200 mg/day) as the sole drug for 1 year and the haemodynamic study was repeated. 4 The blood pressure was reduced approximately 18% both at rest and during exercise, the heart rate approximately 25% and the cardiac output 16% at rest supine and 27% at rest sitting. During exercise the reductions in cardiac output were approximately 20%. The calculated total peripheral resistance was not decreased compared to pretreatment values. The mean arterial pressure-heart rate product was reduced almost 40%. 5 Apart from temporary muscular fatigue during the first weeks, no side-effects were seen. 6 Atenolol is an effective blood pressure lowering drug in mild and moderate hypertension, but the drop in blood pressure is associated with marked reduction in heart rate and cardiac output at rest as well as during exercise.