Passive immunization of rats with an antiserum to synthetic somatostatin caused a 250% elevation of basal serum TSH levels and a nearly 200% increase in TSH-response to TRH. These findings strongly support the concept that pituitary thyrotrophs are regulated by endogenous somatostatin as well as by TRH and thyroid hormones. Serum GH levels in the antiserum-treated rats were considerably higher than those in rats which received normal serum. No clearcut influence on prolactin secretion was observed.