The effectiveness, safety, and pharmacodynamics of repeated doses of intravenous labetalol for rapid reduction of severe hypertension and of subsequent oral labetalol dosing were studied. Twelve patients with severe hypertension were admitted to the hospital after the withholding of antihypertensive therapy for 2 to 14 days. Thirty minutes after an injection of vehicle only, labetalol, 0.25 mg/kg body weight, was injected and followed by repeat injections of 0.5 mg/kg every 15 minutes until the supine diastolic blood pressure (BP) was reduced to less than 90 mm Hg or a total of 3.25 mg/kg had been administered. Twenty-four hours after the last injection, oral labetalol was started at an initial dosage of 100 or 200 mg b.i.d., then increased every 2 days until the standing diastolic BP was less than 90 mm Hg or a maximum daily dosage of 2400 mg was reached. The initial injection achieved mean falls in supine systolic/diastolic BPs of 11/7 mm Hg. Subsequent injections produced additional falls in a dose-related fashion; the mean falls after the last injection (total cumulative dose 2.7 mg/kg) were 40/20 mm Hg. The effect lasted for 12 hours or more in most patients and tended to be biphasic, with one peak at approximately 5 minutes and another much less pronounced peak at about 4 hours. There was no evidence of precipitous falls in BP. All patients were able to ambulate 6 hours after the last injection without symptoms of postural hypotension. Oral labetalol effectively and safely restored and maintained the BP reductions achieved with intravenous labetalol.