1. Oxygen consumption and central haemodynamics were recorded at rest and during exercise in fifty-one men with essential hypertension (W.H.O. stage I) and repeated after 1 year on a single drug: alprenolol (n = 10), atenolol (13) metoprolol (12) and timolol (16).
2. Mean arterial pressure was significantly reduced in all groups at rest (11–18%) and during exercise (5–11%). Heart rate was significantly reduced in all groups (20–28%) at rest and (17–26%) during exercise. Owing to increase in supine resting and exercise stroke volume in the alprenolol and atenolol group, cardiac index decreased less than heart rate-in contrast to the timolol group where cardiac index was decreased 26–32%. The calculated post-treatment total peripheral resistance was significantly increased at rest and during exercise in the timolol group. In the other groups the total peripheral resistance was significantly increased at rest when sitting, but not at rest when supine and during exercise.
3. It is concluded that the major haemodynamic changes induced in subjects with moderate and mild essential hypertension by these different β-receptor blockers are the same, but that minor differences exist with respect to effect upon stroke volume and total peripheral resistance.