Dilevalol hydrochloride, the R-R optical isomer of labetalol hydrochloride, was administered intravenously to subjects with severe hypertension. Twelve subjects with supine diastolic blood pressure of more than 115 mm Hg (mean, 124 +/- 2 mm Hg) were studied. Initial doses of 25 mg of dilevalol administered as a slowly given bolus reduced blood pressure by 14/16 mm Hg. With subsequent additional boluses to a total dose of up to 600 mg, supine diastolic blood pressure was reduced to less than 95 mm Hg in ten of 12 subjects studied (mean reduction, 28 mm Hg). Side effects were minimal and upright blood pressure at the time of reduction of blood pressure to goal was not significantly different from supine blood pressure. Plasma renin activity decreased in 11 of 11 subjects studied. Plasma epinephrine concentrations did not change significantly, whereas plasma norepinephrine concentrations increased 2 1/2-fold, probably reflecting the effect of beta 2-agonism on norepinephrine release. Dilevalol appears to be a safe and effective way of lowering blood pressure short term when intravenous antihypertensive therapy is indicated.