The relation of adrenal function and aging has been the subject of intense interest in recent years. The circadian variations of plasma cortisol have been described in Caucasians, but little information is available on such hormone variations among the Chinese population, especially its changes with age. This study was, therefore, designed to examine the effects of age on the circadian variations of serum cortisol, dehydroepiandrosterone sulfate (DHEAS) and the molar ratio of cortisol/DHEAS in Chinese men, stratified by 10-year age-groups (i.e. men in their 30-60s, aged from 31 to 63 years old). Circadian variations of serum cortisol and DHEAS were documented at 2-h intervals from 8:00 to 22:00 and hourly from 22:00 to 8:00 in 26 healthy Chinese men. We found that the serum levels of both hormones showed a statistically significant circadian rhythmicity in all age-groups. The circadian pattern of serum cortisol was characterized by peaks (04:00-06:00) and troughs (18:00-24:00) occurring approximately 2h earlier than those usually reported in Caucasians. Aging did not significantly influence serum cortisol concentrations, but serum DHEAS levels declined significantly with age: subjects in their 60s had significantly lower levels, and their cortisol/DHEAS molar ratios were significantly higher than those in the younger age-groups.