The endocrine correlates of infertility in primates are not well understood. On the basis of the levels of gonadotropins in the systemic circulation, infertile men can be classified as eugonadotropic, hypergonadotropic, or hypogonadotropic. Men who have “normal” gonadotropin levels and oligospermia are sometimes treated with one of the following: testosterone, human menopausal gonadotropin, or clomiphene citrate. Fertility is improved in a small percentage of the patients thus treated. In syndromes such as Klinefelter's gonadotropins are elevated in the systemic circulation, testosterone levels are low, and there are varying degrees of seminferous tubule failure. The hypogonadotropic syndromes seem to be an extension of the prepubertal state. Infertility in men can be classified according to the amount of testicular damage estimated by testicular biopsy. Generally speaking, the greater the degree of damage to the seminiferous tubule, the greater the quantities of gonadotropin, especially FSH, that can be found in the systemic circulation. In nonhuman primates many behavioral defects contribute to infertility. Testosterone in male rhesus monkeys is secreted on a diurnal schedule. Although the significance of this pattern is not known, differences in the diurnal rhythms of breeders and nonbreeders have been detected. Measurements of gonadal steroids in the systemic serum of fertile and infertile male gorillas did not reveal differences in steroid concentrations that could be correlated with infertility. The possibility remains that some type of infertility in great apes may be related to immunologic factors.