A striking diurnal variation in plasma testosterone concentrations in infantile male rhesus monkeys (Macaca mulatta)

Testosterone (T) was measured by radioimmunoassay in blood samples taken at 09.00 and 21.00 h in infantile male rhesus monkeys maintained in individual cages with their mothers under a controlled photoperiod (lights on 06.00-18.00 h). During the first 6 weeks of life, concentrations of plasma T at night (3.9 +/- 1.7-7.0 +/- 0.8 ng/ml; mean +/- SEM) were approximately 3- to 6-fold greater than those in the morning (0.8 +/- 0.1-2.0 +/- 0.6 ng/ml). The nocturnal elevation in circulating T then declined progressively until approximately 30 weeks of age when plasma T concentrations in the morning and evening reached their nadir and became indistinguishable. Bilateral orchidectomy at 1 week of age completely abolished the nighttime elevation in plasma T concentration. This striking diurnal variation in testicular T secretion in the infantile rhesus monkey appears to be similar to that in the fully adult animal. These findings provide further evidence in support of the view that, in this species, the hypothalamic-pituitary apparatus that governs testicular function is fully mature by the neonatal stage of development.