AbstractIntraperitoneal injection of mammalian thyrotropin‐releasing hormone (TRH) caused hypertrophy of possible thyrotropic cells in the pituitary gland of the teleost,Chasmichthys dolichognathus.The epithelial cells of the thyroid follicles also appeared hypertrophic after the TRH injection. On the other hand, in the hagfish,Eptatretus burgeri, TRH injection had no effect either on the cytological appearance of the adenohypophysis or on the thyroidal activity in terms of serum thyroxine concentration and epithelial cell heigh