In 20 patients with essential arterial hypertension, grade II or III, the haemodynamic effect of a fixed combination of antihypertensive drugs (bemetizide, triamteren, bupranolol and dihydralazine) was studied by percutaneously-introduced flow-guided catheter in the pulmonary artery before, after three weeks and at the end of a six-month period of treatment; systemic arterial blood pressures were measured by the cuff method. Mean pressures, both at rest and on exercise were significantly and persistently lowered. Mean blood pressure at rest fell from 184/110 mmHg before treatment to 167/98 mmHg after three weeks, and to 158/93 mmHg after six months. The initial blood pressure on exercise was 226/129 mmHg, which fell after three weeks to 199/113 mmHg and after six months to 190/106 mmHg. At the same time, pulmonary artery pressure (as a measure of preload) fell by 23% at rest and 30% on exercise. Exercise tolerance of the patients rose by 28%. Pressure-heartrate product fell by 18%, as a pointer to an additional favourable effect on left ventricular oxygen consumption.