It is well recognized that weight reduction produces the suppression of serum LH but not FSH level in rodents. In order to clarify the mechanism by which the discrepancy between LH and FSH levels is brought about, the influence of weight loss on the pituitary function was explored using female rats. The changes of the pituitary response to GnRH and the basal secretion of gonadotropins with progressive weight loss were investigated by in vitro short-term incubation of the pituitary gland after prolonged weight loss in female Wistar rats. On the first day of diestrous and until day 14 of the diet, GnRH induced LH and FSH release from the pituitary and a decrease in pituitary content of them, but the total amount of gonadotropin in culture medium and pituitary tissue was not affected. On day 30 of the diet, the decrease in pituitary content disappeared. On day 60 LH release disappeared, whereas pituitary FSH and the total amount of gonadotropins were increased by GnRH. Non-stimulated FSH but not LH secretion per mg of pituitary was augmented during dieting. The data indicate that pituitary responsiveness to GnRH and non-stimulated FSH release were modified by weight loss: the LH-releasing action of GmRH was diminished, the gonadotropin-synthesizing action of GnRH was augmented, and non-stimulated FSH release was increased.