Testicular exocrine and endocrine function was monitored in adult male rhesus monkeys maintained for up to 4 years in a controlled environment isolated from seasonal changes in light, humidity and temperature and from female animals.Marked circannual variations were found in both functions. Spermatogenesis and exocrine function were maximal in the autumn and winter months as indicated by histological studies and the measurement of testicular volume, frequency of spontaneous and provoked ejaculations and in sperm output. The serum level and metabolic clearance rate and production rate of testosterone were also maximal during this period as were the serum concentrations of dihydrotestosterone, cortisol and biologically active luteinizing hormone. By contrast the serum prolactin and dehydroepiandrosterone levels showed an inverse pattern, achieving their highest levels in spring, during the period of reduced testicular function.Since such marked circannual variations in testicular and pituitary functions persist in monkeys isolated from external environmental influences, the existence of an inherent regulatory mechanism can be postulated. Such distinct variations in testicular function must be taken into consideration when using the male rhesus monkey as an experimental model for human reproductive function.