1. Labetalol (AH 5158) is a new drug which has both α- and β-adrenoreceptor antagonist activity. Labetalol has been compared with propranolol and placebo for the treatment of mild to moderate hypertension (150/100 to 189/114 mmHg), in a double-blind cross-over trial with three 10 week treatment periods.
2. Both drugs reduced blood pressures in nine of the ten patients who have completed the trial. Relative to placebo labetalol caused decreases in supine pressures similar to those caused bypropranolol (average −10/ −10 and −10/ −9 mmHg respectively), with a greater effect on standing (labetalol −20/ −19; propranolol −8/−12 mmHg; P < 001) and after exercise (labetalol −27/−18 and propranolol −16/−14 mmHg).
3. Labetalol and propranalol caused a similar fall in supine pulse rate (−11/min and −13/min respectively), but there was less bradycardia on standing (labetalol −11/min; propranalol −18/ min; P < 0001) and after exercise (labetalol −17/min; propranolol −23/min; P < 0·001).
4. The average dose ratios of labetalol to propranolol was 2·4:1 (w/w).
5. These results suggest that the effect of labetalol on blood pressure involves both α- and β-adrenoreceptor antagonist properties.
6. Further evaluation of labetalol is warranted as it may have some advantages in respect of producing less bradycardia, improved peripheral blood flow and less effect on airways resistance than ‘pure’ β-receptor-blocking agents.