The in vivo incorporation of [35S]cysteine into hypothalamic somatostatin-28 was found to be substantially below normal in hypophysectomized rats. A smaller reduction in label incorporation into arginine vasopressin was also observed, while incorporation into acid-precipitable protein was normal. The diminution in somatostatin biosynthesis presumably reflects the absence of pituitary growth hormone secretion, while that in vasopressin synthesis may reflect the loss or disruption of vasopressin-producing cells.