Diacetolol is the major metabolite of the β-adrenoceptor blocking agent, acebutolol. In vitro, the β-adrenoceptor blocking potency of diacetolol was less than that of acebutolol but its cardioselectivity (atrial relative to tracheal tissue) was greater. Diacetolol had weak intrinsic sympathomimetic activity, and no significant membrane-stabilizing activity. It did not restore sinus rhythm to anaesthetized dogs with ouabain-induced arrhythmias but was similar to acebutolol in preventing arrhythmia induced by adrenaline/methylchloroform in anaesthetized cats. Study of diacetolol pharmacokinetics and bioavailability suggests either a first-pass effect or incomplete absorption of diacetolol after oral administration. Diacetolol plasma half-life after oral administration is about 12 h and is not dose-dependent. Diacetolol possesses significant antihypertensive action against moderate essential hypertension in man. Its antihypertensive effect is associated with a reduction in the heart rate and a decrease in plasma renin activity.