1. Labetalol was administered to 18 hypertensive patients for an average duration of 2·44 weeks, with an average final daily dose of 1·65 g.
2. Labetalol decreased resting heart rate by 16% and maximal exercise heart rate by 21%; the phenylephrine-induced rise of systolic brachial artery pressure was reduced by 36%.
3. During labetalol brachial artery pressure was lowered by 29/15 mmHg in the recumbent position, by 41/23 mmHg at rest sitting, and by 53/23 mmHg at maximal exercise; total peripheral resistance was not significantly affected at rest recumbent, but was reduced at sitting and at exercise; cardiac output decreased in all conditions.
4. Labetalol reduced mean pulmonary artery and capillary wedge pressures only in the sitting position. Pulmonary vascular resistance remained unchanged.
5. The drug produced significant decreases of plasma renin activity and of plasma aldosterone concentration.