The haemodynamic effects of labetalol were studied in patients with essential hypertension after acute and long-term administration. In the acute study 50 mg of labetalol was given i.v. to 10 patients. Eight of them were re-examined after three to four months of oral treatment with labetalol 300 mg t.i.d. Measurements were made at supine rest, after five minutes in the upright position, and after five minutes' supine bicycle exercise at 50 and 100 watt work loads. In the acute study cardiac output was unchanged except at the work load of 100 watt when it decreased 18%. The mean blood pressure decreased under all conditions, 11.6 mm Hg at supine rest, 22.3 mm Hg in the upright position and 15.9 mm Hg and 16.9 mm Hg at the two work loads. Heart rate was unchanged at supine rest, and reduced by 10% in the other situations. Total peripheral resistance was reduced significantly at rest in the supine and upright positions, but not during exercise. Pulmonary artery pressure was unchanged. In the long-term study cardiac output was unchanged except of the heavy work load when it decreased 11%. Mean blood pressure was reduced significantly by 14.6 mm Hg at supine rest, 16.8 mm Hg in the upright position and by 13.9 mm Hg and 13.4 mm Hg, respectively, at the two work loads. Heart rate was reduced significantly in all conditions by 10-14%. Total peripheral resistance decreased significantly at rest, but not during exercise, and pulmonary artery pressure was unchanged.