The effect of once daily low-dose carvedilol on circadian variations of blood pressure (BP) was studied in Japanese patients with mild or moderate essential hypertension. Thirty-one patients were admitted to hospital whose BP was 150/90 mm Hg/day or greater and they participated in the study. After a placebo period of 1 week, 5 or 10 mg carvedilol was given once daily in the morning for 3 to 7 days, and if BP reduction was not sufficient, the dose was increased to 20 mg daily. The blood pressure variation was monitored before and 1, 2, 4, 6, 8, 10, 12, and 24 h after administration of the drug on the last day of placebo and final dose of carvedilol. Of the 31 cases receiving carvedilol once daily, cumulative effectiveness (13 mm Hg reduction in mean BP) was 48.4% at 10 mg/day and 54.8% at 20 mg/day. Both systolic and diastolic pressures decreased significantly and heart rate decreased slightly. There was no significant difference between the standard deviations of BP on the last days of the control period and the carvedilol treatment. The difference between maximum and minimum BP during the day was not significant between the two periods. Circadian variations of heart rate were also not significantly different for the two periods. This indicates that carvedilol did not have any effect on circadian variations of BP and heart rate. The present study also suggests that low-dose carvedilol once daily may be effective in the treatment of hypertension.