ZIAK is a combination of two antihypertensive agents bisoprolol and hydrochlorothiazide. Bisoprolol is a beta 1-selective beta-adrenoceptor antagonist with low beta 2-receptor affinity. It has no intrinsic sympathomimetic activity nor membrane-stabilising properties. It reduces blood pressure and by blockade of the cardiac beta 1-receptors reduces the heart rate and depresses plasma renin levels. Bisoprolol is rapidly absorbed after oral administration in man and excreted predominantly via the urine as unaltered substance and metabolites. In man 50% of a dose is metabolised in the liver while the other 50% is eliminated unchanged via the kidneys. None of the metabolites have beta 1-receptor blocking action. The plasma elimination half-life is 10-12 hours resulting in a duration of action of 24 hours. Because of its moderate hepatic metabolism it is subject only to a very small hepatic first pass metabolism. Therefore bisoprolol displays a high bioavailability of 90% after an oral dose. Hydrochlorothiazide is a benzothiadiazine diuretic. Thiazides inhibit sodium and chloride reabsorption in the kidney tubules and produce a corresponding increase in potassium excretion. It is readily absorbed from the gastro-intestinal tract. It is reported to have a bioavailability of about 65 to 70%; the plasma half-life is about 5 hours with a subsequent longer terminal phase; its biological half-life is up to about 15 hours. It is excreted unchanged in the urine. ZIAK is indicated for the treatment of hypertension.