Age-related changes in the 24-h plasma concentrations of (A) testosterone, (B) dehydroepiandrosterone sulphate (DHEAS), and (C) cortisol in male rhesus macaques. The panels depict mean hormone profiles from 10 adult (~10 years, shown in red) and 10 aged (~26 years, shown in black) animals, and the horizontal light and dark bars correspond to the 12L:12D lighting schedule; note that the profiles have been double plotted to facilitate observation of day–night differences. Each hormone showed a distinct 24-h rhythm, with a peak occurring either during the night (testosterone) or early in the morning (cortisol and DHEAS). Both testosterone and DHEAS showed a significant age-related attenuation in the plasma levels whereas cortisol showed a significant increase. Figure adapted from Urbanski and Sorwell (9).