Testicular size, testosterone level, and body weight of 15 male Macaca radiata were studied over a 19-month period. Seasonal biological variations corresponding to the species’ reproductive cyclicity were observed. Adults experienced synchronous seasonal changes in testis size, testosterone, and body weight. Adolescents displayed rapid maturational increases and seasonal variations in testicular size, continuous weight gains accelerating during the breeding period, and a marked seasonality of testosterone. Juveniles displayed continuous maturational increases in testicular size and body weight, and a limited but significant seasonality of testosterone. Extent of seasonal variation in all three parameters increased with age.