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Clinically healthy diurnally active young adult women were studied during the same season (March) at the Universities of Kyushu (Fukuoka City, Japan) and of Minnesota (Minneapolis, U.S.A.), under comparable conditions, except that the habitual diets were not changed. The subjects (20 Japanese and 16 Americans of mixed Caucasian background) were studied over a single 24-hr span. Urine was collected at 4-hr intervals. A circadian rhythm in total urinary norepinephrine excretion showed similar characteristics in Japanese and Americans. In epinephrine excretion, the Japanese women showed a statistically significantly higher amplitude with higher peak values, but no statistically significant difference in the rhythm-adjusted mean. This intergroup difference is strictly time dependent; it does not come to the fore in urine samples covering the nocturnal rest span of the subjects.

A circadian rhythm of plasma activity (PRA) was demonstrated for both Japanese and North American women, the latter mostly Caucasians of mixed ethnic origin. The results were based on blood samples withdrawn at 4-hour intervals during a 24-hour span (in March 1978) from 20 subjects from Fukuoka (average age 20.4 +/- 0.1 years) and 16 subjects from Minneapolis (average age 20.2 +/- 0.4 years). The rhythms in the two populations showed similarities in some characteristics and differences in others. The timing of high values, i.e., of acrophases, objectively assessed by curve-fitting (and of corresponding 95 per cent confidence limits) was at 07(36) (05(00), 10(16) and 06(32) (03(00), 10(00) for Japan and USA, respectively. As objective measures of the extent of predictable rhythmic change mean amplitudes, in nanograms per milliliter per hour (ng/ml/hour), were similar (0.31 and 0.32); a statistically significant difference (P less than 0.05) was found in mean amplitudes expressed as percentage of the rhythm-adjusted average. Mean rhythm-adjusted average values (mesors) were lower in women from Japan than in those from the United States: (1.64 +/- 0.14 and 2.39 +/- 0.23 ng/ml/hour, respectively; P less than 0.01). A statistically significant difference in dietary salt, indicated by differences between the Japanese and North American women in the urinary excretion of sodium and chloride (P less than 0.05), almost certainly contributed to these results.