50 patients with confirmed hypertension were treated with endralazine, a new peripheral vasodilator, in addition to the beta-blocker, pindolol, to which they had not responded adequately. The blood pressure was lowered from 173/115 mm Hg to 143/87 mm Hg in the 34 slow acetylators and from 175/111 mm Hg to 140/84 mm Hg in the 16 fast acetylators. The dosages of both pindolol and endralazine were the same in both groups. It is concluded that the acetylator phenotype does not affect the therapeutic efficacy or dosage requirement of endralazine. It is suggested that this is because endralazine is metabolised mainly by hydrazone formation and only to a minor extent by acetylation.