The cardiovascular effects of a new antihypertensive agent, prizidilol hydrochloride (SK&F 92657), have been evaluated in a group of 13 patients with mild to moderate essential hypertension, both at rest and during mental stress and isometric or dynamic exercise. After 6 weeks of therapy (200-300 mg of prizidilol b.i.d.) both systolic and diastolic blood pressure values at rest were significantly lower than those recorded before treatment and during placebo (with an average decrease of about 22/11 mm Hg in the supine position). Heart rate at rest was slightly reduced. Cardiovascular responses to dynamic exercise and mental stress were not prevented by prizidilol, but blood pressure and heart rate levels around which variation occurred were lowered to a significant extent. During isometric exercise prizidilol did not protect against a rise in diastolic blood pressure. Blood pressure and heart rate reduction both at rest and during stimulation were not related to plasma drug concentrations. Side-effects were very rare. It is concluded that prizidilol significantly reduces blood pressure both at rest and during mental stress and physical activity with only sight effects on heart rate, and with a very simple therapeutic scheme.