Day and night levels of hormones in male rhesus monkeys kept under controlled or constant environmental light.

The marked differences between day and night levels of testosterone, cortisol, prolactin and bioactive luteinizing hormone observed in adult male rhesus monkeys maintained at 12 h light: 12 h dark schedule are not changed by exposing monkeys to constant illumination for a period of up to 15 days. These findings suggest that the photoperiodicity of environmental light may not be the principal determinant for the occurrence of the diurnal rhythms observed in the circulating levels of the 4 hormones studied.