Eight patients with a diastolic blood pressure greater than or equal to 100mm Hg when treated with a diuretic and a beta-blocker participated in a randomised crossover study comparing the haemodynamic effects of adjunctive therapy with pinacidil or hydralazine. The vasodilator dose was increased until the diastolic blood pressure was less than 90mm Hg or the maximum dosage, hydralazine 100mg twice daily, or pinacidil 50mg twice daily, was reached. Treatment continued for 3 to 6 months and a haemodynamic study was performed. After washout, the patients received the alternative treatment. In the upright position, during supine rest and during isometric as well as dynamic exercise, pinacidil lowered blood pressure more effectively than hydralazine. No differences between the 2 treatments were found in heart rate, stroke index, cardiac index, end systolic wall stress or glomerular filtration rate. Pulmonary mean and wedge pressure were lower during treatment with pinacidil. Forearm blood flow was higher and forearm vascular resistance lower during treatment with pinacidil. Cardiac contractility, judged from the systolic time interval ratio PEP: LVET, was lower during treatment with pinacidil compared with hydralazine. The median daily dose of pinacidil was 50mg and that of hydralazine 200mg. It was also noted that during long term treatment, pinacidil seemed more effective in reducing blood pressure than hydralazine.