The obese (genotype ob/ob) mouse is known to have a genetically acquired form of hypogonadotropic hypogonadism. Because the animal has multiple abnormalities of endocrine systems, including impaired growth, obesity, and temperature regulation, it has been assumed by many to have a hypothalamic defect. In an attempt to separate hypothalamic from pituitary dysfunction, an acute LHRH response test was administered to obese and lean littermates. In both lean and ob/ob adult male animals, a bolus of LHRH increased serum LH concentration, but the concentrations attained were 2-fold greater in the lean animals. To determine if the defect in the obese animals was the result of chronic understimulation of the pituitary gland due to LHRH deficiency, LHRH was administered three times daily for 20 days, after which the acute LHRH test was repeated. After 20 days of LHRH treatment, the obese animals remained less responsive than the lean mice to a bolus of LHRH. Surprisingly, in both lean and ob/ob animals, chronic LHRH treatment resulted in a smaller LHRH response to acute LHRH than in the untreated animals. This latter finding was not due to the development of anti-LHRH antibodies. Ob/ob mice have impaired response to LHRH that is not correctable by chronic LHRH administration. These data are consistent with a defect in pituitary function in these animals.