Isradipine, 2.5 mg twice daily and placebo were administered for 4 weeks to 11 untreated essential hypertensives in a double-blind, random order, crossover study. At the end of each phase patients were assessed by 36 h of continuous intra-arterial blood pressure monitoring, surface electrocardiography, and measurement of endogenous creatinine clearance, serum biochemistry and plasma renin activity. Arterial pressure was significantly reduced by isradipine, with mean reduction in systolic blood pressure of 11.0 mmHg and diastolic pressure of 13.2 mmHg over 24 h. There was no significant change in heart rate, endogenous creatinine clearance, serum biochemistry or plasma renin activity.