PIP: Endocrinological aspects of male infertility are reviewed, beginning with the physiological interrelationship between the testis and the hypothalamo-hypophysical unit. The failure of the pituitary to secrete follicle stimulating hormone (FSH) and luteinizing hormone (LH) results in disruption of testicular function and infertility. However, in men presenting with infertility, gonadotropin deficiency accounts for less than .5% of the causative factors. If deficiencies in FSH or LH are proved, a hypothalmic or pituitary lesion should be sought. Nevertheless, mreasurements of FSH, LH, and prolactin are useful tests in the management of male infertility. Of greatest importance is measurement of serum FSH, which provides a useful index of the state of the seminiferous epithelium when the concentration is related to sperm density: high concentration associated with severe oligospermia or azoospermia usually denotes untreatable infertility. Elevated LH and low testosterone levels have been found in about 30% of men with severe degrees of testicular damage and are indicative of interstitial cell failure. Prolactin measurements are mainly associated with impotency rather than infertility. Hormonal treatment of male infertility is often indicated. For example, replacement therapy for gonadotropin deficiency is successful. Androgen injections of testosterone esters will suppress spermatogenesis, so when treatment is stopped, sperm counts will rebound to concentrations greater than pretreatment levels. That endocrine factors can potentiate testicular damage is postulated based on the measurement of FSH as an indicator of seminifirous tubule disruption resulting in disruption of spermatogenesis.