Twenty-four-hour ambulatory blood pressure (BP) was monitored in 540 healthy Japanese adults whose casual BP was less than 140mmHg in systole and less than 90mmHg in diastole. A total of 304 men and 236 women, ranging in age from 20 to 79 years, with a mean of 40.9±12.8 years, were studied. BP was measured every 30min using ABPM-630 (Nippon Colin Co.) for 24h in 395 subjects and for 48h in 145 subjects. Periodic regression curves were obtained and the coefficients of determination (R²) was calculated for each periodic component. The periodic regression curves by age and sex were tested statistically for differences in terms of the level (mean value) and the pattern (amplitude and acrophase) of the curves using periodic analysis of covariance (PERCOVA). The periodic regression curve composed of 24- and 12-h periods was fitted satisfactorily to the measured BP values (R²=0.97 for systolic BP and 0.98 for diastolic BP). The level of the periodic regression curve increased significantly with increasing age, being significantly higher in males than in females. The pattern of BP curve did differ among some age groups but not significantly between the sexes.(Hypertens Res 1992; 15: 127-135)