[3H]Guanidinoethylmercaptosuccinic acid (GEMSA) autoradiography demonstrates the particulate form of a carboxypeptidase B-like peptide processing enzyme, enkephalin convertase (EC 3.4.17.10), in the rat pituitary and adrenal glands. The maximal number of binding sites (Bmax) for [3H]GEMSA is 20 pmol/mg protein in the intermediate lobe of the pituitary, 12.0 pmol/mg protein in the posterior pituitary lobe, 15 pmol/mg protein in the anterior pituitary lobe, 5.8 pmol/mg protein in the adrenal medulla, and less than 0.3 pmol/mg protein in the adrenal cortex. The labeling pattern is homogeneous within each of these regions. Subcellular fractionation of the bovine adrenal medulla demonstrates that [3H] GEMSA-binding sites are localized to chromaffin granules. In Brattleboro rats and dehydrated rats, the level of posterior pituitary [3H]GEMSA binding is less than 25% of that in control animals. This decrease is abolished by arginine vasopressin treatment of Brattleboro rats or rehydration of dehydrated rats. There are no changes in [3H] GEMSA binding in the supraoptic nucleus or magnocellular portion of the paraventricular nucleus of the hypothalamus under any of these conditions, suggesting that the alterations observed in the neurohypophysis result from an increased rate of loss of enkephalin convertase. The level of anterior pituitary enkephalin convertase is unchanged by dehydration, adrenalectomy, or dexamethasone or in Brattleboro rats. [3H]GEMSA labeling in the intermediate pituitary lobe is unaffected by dehydration and haloperidol treatment and in Brattleboro rats. The adrenal medullary enzyme is not altered by reserpine, hypophysectomy, or splanchnic denervation or in Brattleboro rats.