Prolactin inhibits thyroxine-induced tail regression, a metamorphic event which has been unequivocally shown to be caused directly by thyroxine. On the other hand, thyroxineinduced pancreatic regression, and acid phosphatase increase and biochemical differentiation in the liver are not inhibited or counteracted by prolactin treatment. Therefore, prolactin does not antagonize the entire process of metamorphosis. These studies also indicate that some metamorphic events are not directly induced by thyroxine (Endocrinology85: 1213, 1969)