A double-blind parallel group study was conducted to examine the effects of oral labetalol, in doses from 100 to 800 mg BID, and propranolol, 40 to 320 mg, in patients with mild to moderate hypertension. The doses of labetalol (n = 74) and propranolol (n = 79) were titrated weekly to achieve a sitting diastolic blood pressure (DBP) of less than 90 mmHg or at least a 10-mmHg decrease from placebo baseline on two consecutive visits. A 2-month fixed-dose maintenance phase followed in which a diuretic could be added if the sitting DBP was greater than or equal to 100 mmHg on maximum doses of either drug. BP and heart rate were measured 8-12 hours after a dose in the sitting and standing positions. Labetalol was significantly more effective at the end of monotherapy than propranolol was in lowering both the sitting (p less than .05) and standing (p less than .04) DBP. The reduction in the systolic, although more pronounced for those on labetalol, was not significantly different; 53% of patients had a "good" response to labetalol compared with 30% of the propranolol group. Propranolol significantly (p less than 0.01) lowered heart rate compared with labetalol. Nine patients in the labetalol group and 10 in the propranolol group required a diuretic. The decrease in BP after the addition of a diuretic was comparable. Changes in plasma lipids were not significant, but HDL increased 9% with labetalol and decreased 2% with propranolol. Triglycerides increased 25% with labetalol and 31% with propranolol.(ABSTRACT TRUNCATED AT 250 WORDS)