Evidence of a seasonal cycle in testicular size in a population of free-ranging Rhesus monkeys is presented. Tests are large during the season in which copulations are observed, and small during the season in which young are born. The color of the sex skin of the male is brightest during the season of large testes, and least intense during that of small testes: an accompaning hormonal cycle is suggested. Implicit assumptions that male monkeys are sexually receptive uniformly throughout the year should be reexamined.