The antihypertensive effects of intravenous labetalol were evaluated in 59 patients with hypertensive crises or severe hypertension in need of rapid lowering of blood pressure in a multicenter study. Patients appearing with a supine diastolic blood pressure 125 mm Hg or greater, or a supine systolic blood pressure of more than 200 mm Hg received an initial mini-bolus injection (20 mg) of labetalol. This was followed by repeated incremental doses of 20 to 80 mg given at 10 minute intervals to achieve a supine diastolic blood pressure of less than 95 mm Hg or decrease 30 mm Hg or greater, or a satisfactory decrease in systolic blood pressure. Patients were stratified into those who had taken antihypertensive medication within 24 hours and those who had not. The initial mini-bolus injection caused rapid but not abrupt reduction in blood pressure; the baseline mean blood pressure decreased 23/14 mm Hg. Further injections were needed in the majority of patients (mean: 197 mg). The blood pressure reduction after the last dose of labetalol was 55/33 mm Hg. In pretreated patients and in those who had no medication for 24 hours prior to the intravenous labetalol, the response was similar. Heart rate decreased 10 beats per minute in the total population. In patients pretreated with beta-adrenergic blockers, blood pressure response was similar to that in the total group (59/35 versus 55/33 mm Hg), but heart rate remained essentially unchanged. The dose required to achieve the therapeutic effect was less in pretreated patients than in untreated patients, but the duration of action was shorter. No serious adverse effects were encountered even in patients with concomitant diagnoses of acute left ventricular failure, myocardial infarction, stable congestive heart failure, atrial fibrillation, angina pectoris, acute stroke, transient ischemic attack or encephalopathy. Labetalol is a safe and effective treatment for a rapid blood pressure reduction in hypertensive emergencies.