Fig. 3.

Characteristic plasma testosterone rhythm in adult male rhesus macaques, revealed by remote serial blood sampling every 30 min for 24 h. a Double-plotted actogram from a representative male rhesus macaque, showing diurnal activity that is entrained to the 12L:12D light cycle (indicated by the white and black horizontal bars). b Double-plotted mean plasma testosterone levels from 10 animals (±SEM). Note that the 24-hour rhythm is superimposed on ultradian testosterone level fluctuations that stem from an underlying pulsatile pattern of LH release. Note also that in contrast to the plasma cortisol and DHEAS rhythms, which show a peak in the early morning (cf. fig. 1), the testosterone peak occurs during the night when the animals are asleep.