The antihypertensive effect of isradipine was assessed by repeated 24-h ambulatory blood pressure monitoring. Using an SPS device (Sandoz Pharma, Basel, Switzerland), monitoring was carried out in 10 male patients with mild essential hypertension (1) after a placebo period, (2) after six months, and (3) after 12 to 13 months of treatment with isradipine (average dose 2.5 mg twice daily). Mean 24-h blood pressure decreased significantly after both periods 2 and 3 (from 148/93 mm Hg to 137/87 and 130/85 mm Hg, respectively). The total number of hypertensive systolic and diastolic blood pressure values also decreased. The normal circadian blood pressure curve was preserved, showing the reduction throughout the 24-h period, and the early morning rise in blood pressure was markedly blunted. These results indicate that isradipine has a favorable effect on the 24-h blood pressure profile that persisted throughout six and 12 months of antihypertensive therapy.