We evaluated the effect of barnidipine, a dihydropyridine calcium antagonist, administered once daily in the morning in a dose of 5, 10, or 15 mg on ambulatory blood pressure (BP) in 34 patients (51.3+/-9.6 years). Hypertension was diagnosed based on the clinic BP. The patients were classified into groups according to the ambulatory BP: group 1, dippers with true hypertension; group 2, nondippers with true hypertension; group 3, dippers with false hypertension; and Group 4, nondippers with false hypertension. Barnidipine reduced the clinic systolic BP (SBP) and diastolic BP (DBP) in all groups and significantly reduced the average 24 h ambulatory BP (133.0+/-16.5/90.7+/-12.3 mm Hg v 119.7+/-13.7/81.8+/-10.3 mm Hg, P < .0001 for both SBP and DBP). Barnidipine significantly reduced the daytime ambulatory SBP in groups 1, 2, and 3, but not in group 4, and significantly reduced daytime ambulatory DBP in group 1 but not in groups 2, 3, and 4. Barnidipine significantly reduced the nighttime ambulatory SBP only in group 2 and the nighttime ambulatory DBP in groups 2 and 4. Once-a-day administration of barnidipine influenced 24 h BP on true hypertensives (the ratio of the trough to peak effect > 50%), but had minimal effect on low BP such as the nocturnal BP in dippers and the ambulatory BP in false hypertensives. These findings suggest that barnidipine can be used safely in patients with isolated clinic ("white coat") hypertension and in those with dipping patterns of circadian BP variation whose nocturnal BP is low before treatment.