Reproductive function is impaired in the genetically obese (C57 B1/6J) ob/ob mouse. Serum LH, FSH, and testosterone concentrations were assessed in male ob/ob and lean littermates from 39 to 78 days of age. The lean animals demonstrated a three-fold rise in serum LH between 39 and 45 days of age that preceded a steep increase in serum testosterone which peaked at age 70 days. The obese animals did not demonstrate this LH rise; serum testosterone levels were low and had a blunted increase with age that paralleled that of normal animals. Serum FSH was lower than normal at all ages in the obese mice. The ventral prostrate and testes were small in the ob/ob mice. The castration of adult animals resulted in increased serum concentrations of both LH and FSH, with higher levels attained in the lean animals. Fiftyfour- day-old castrated lean and obese mice were treated with testosterone for 15 days. Measurements of serum LH and FSH after 8 and 15 days of treatment demonstrated a marked sensitivity in the ob/ob animals to feedback inhibition of gonadotropins. This finding suggested persistent immaturity of the hypothalamic-pituitary axis in obese mice. These studies indicate that the hypogonadism of the ob/ob mouse is the result of altered hypothalamic- pituitary function. (Endocrinology98: 1359, 1976)