Sixty peripartum patients with diastolic blood pressures (BP) 110 mmHg or higher were randomized in a 2:1 ratio to receive repeated intravenous injections of either labetalol (20-80 mg) or hydralazine (5 mg) until the diastolic BP was below 100 mmHg. There were four treatment failures in the labetalol group (N = 40) and none in the hydralazine group (N = 20). Hydralazine lowered mean arterial pressure (MAP) more than did labetalol (33.3 +/- 13.2 versus 25.5 +/- 11.2 mmHg; mean +/- SD), but labetalol had a more rapid effect. There was considerable interpatient variability in the dose of labetalol required to control BP, which could not be predicted by any clinical characteristic before therapy. The duration of action also varied in the labetalol group, with the shortest duration occurring in those patients who required the highest dosage for BP control. No significant fetal or neonatal problems ascribable to drug treatment were noted in the 13 instances in which labetalol was given before delivery. However, fetal distress occurred in two of the six cases involving antenatal hydralazine. We conclude that labetalol appears to be a safe and effective alternative to hydralazine for treating hypertension in the peripartum period, but serious rare side effects have not yet been quantified.