Young rhesus monkey females (Macaca mulatta) demonstrate the well-defined circadian rhythm in activity of erythrocyte SOD with maximum at 10.00 h and minimum at 22.00 h. However, neither GSH-Px nor GR demonstrated any significant circadian changes, contrastingly to SOD. The diurnal changes in the SOD activity tightly correlate with the diurnal changes in the levels of cortisol and DHEAS in the animals' blood plasma. With aging, these circadian rhythms of SOD, cortisol and DHEAS are smoothed out although the correlation between the diurnal changes in cortisol and SOD still maintains even for old animals. These results suggest that corticosteroids play an essential role in regulation of the SOD activity and that the reliability of the hormonal regulation decreases with aging.