The ultrastructural changes induced by partial hepatectomy in the STH cells of mice were examined. These changes suggest an increased secretory activity of STH cells. Therefore, several experimental procedures elucidating the possible mechanism of this effect were undertaken. Normal mice were starved for 52 h or treated with insulin, glucagon, or cAMP. Serum glucose level measurements as well as electron microscopy of pituitary STH cells were carried out in each group of animals. It was found that STH cells of glucagon and cAMP-treated mice showed some of the ultrastructural features observed in partially hepatectomized animals.Centro de Endocrinología Experimental y Aplicada