This study was designed to clarify the relationship between the antihypertensive effects of the calcium antagonist nilvadipine, and circadian changes in blood pressure. Based on measurements using an ambulatory blood pressure monitoring system (ABPM), 17 outpatients with untreated essential hypertension were divided into two groups: a sustained hypertensive group (with a fall in blood pressure during sleep < 10%, n = 7) and a waking time hypertensive group (with a fall in blood pressure during sleep > or = 10%, n = 10). During treatment with nilvadipine (8 mg/day, > or = 2 weeks), patients were reexamined by ABPM. The antihypertensive effect of nilvadipine was significantly and negatively correlated with the night time fall in blood pressure: this effect was significantly greater in the sustained hypertensive group than in the waking time hypertensive group. These data suggest that the long acting calcium antagonist nilvadipine has more potent antihypertensive effects in patients with sustained hypertension ("nondippers") than in those whose hypertension lessens during sleep ("dippers").