We examined the effects and mechanisms of action of ACTH and ACTH fragments on gonadotropin secretion in immature rats. ACTH administered by daily injection or continuous infusion (osmotic minipumps) attenuated the postcastration rise in serum LH. Pituitary LH concentration was either unchanged or increased in AGTH-treated rats and pituitary sensitivity to gonadotropin-releasing hormone (GnRH) was reduced by ACTH treatment. A fragment of ACTH (ACTH 4-10), which is less steroidogenic, did not alter levels of serum LH, and ACTH did not reduce LH secretion in adrenalectomized castrates. Serum and pituitary concentrations of prolactin were normal in ACTH-treated animals. These studies demonstrate that the suppression of gonadotropin secretion by ACTH is mediated by the adrenal gland. This mechanism causes a decreased pituitary sensitivity to GnRH, but LH synthesis does not appear to be affected. Prolactin does not play a role in this mechanism.