Michael, R. P., Setchell, K. D. R., Plant, T. M. 1974. Diurnal changes in plasma testosterone and studies on plasma corticosteroids in non-anaesthetized male rhesus monkeys (Macaca mulatta). Journal of Endocrinology 63, 325-335.

Authors evaluated plasma corticosteroid concentrations in male rhesus macaques who were trained to enter voluntarily a restraining apparatus and permitted venipuncture without showing signs of stress. Basal cortisol values were 30% lower than previously reported for different, untrained animals.