It is generally believed that the meta-adenohypophysis of the teleost pituitary is homologous with the pars intermedia of the tetrapod pituitary and is responsible for the secretion of a melanophore-stimulating hormone (MSH). This conclusion is based on comparative anatomical and histological studies and the fact that in several teleosts the concentration of MSH in the posterior half of the pituitary is considerably higher than in the anterior half (Pickford & Atz, 1957; Kent, 1959). The present work provides more direct evidence that the cells of the meta-adenohypophysis are the site of MSH synthesis. Adult Salmo gairdneri, 27–30 cm. in length, average weight 200 g., were used. MSH was assayed in vitro by the Anolis skin test (Burgers, 1961). The Anolis unit—that concentration of MSH which will just cause melanophore dispersal in isolated fragments of Anolis skin—was equivalent to 0·004 μg./ml. Intermedin (Choay Laboratories, Paris). In a preliminary investigation, the concentration