HYPOGONADOTROPIC hypogonadism (HH) is a clinical syndrome occurring in both sexes which has long puzzled clinicians due to the apparent paradox of nonfunctioning gonads in the face of normal or only slightly lowered levels of circulating gonadotropins. Using frequent sampling of gonadotropin levels as an index of hypothalamic GnRH secretion, we have examined the hypothesis that this group of disorders represents a spectrum of abnormal patterns of the pulsatile release of endogenous GnRH. After a broad, normative data base was established in both men and women for purposes of comparison, it appears that quantifiable abnormalities of GnRH secretion are discernible in both males and females with HH. These abnormalities include a total absence of GnRH secretion, defects of the amplitude and frequency of its secretion, and altered bioactivity of the gonadotropins released. In addition, physiological regimens of hypothalamic replacement therapy with exogenous GnRH, which are fashioned to mimic the normal frequency of endogenous GnRH secretion, result in complete normalization of reproductive function and fertility in hypogonadotropic subjects of both sexes.