ABSTRACT Carassius auratus were injected with pituitary exophthalmos-producing substance and the subsequent histological changes studied. Oedema, infiltration of macrophages in the retrobulbar area, muscle swelling and splitting, and increase of mucinous substance were noted. Since such changes have been reported for exophthalmic human patients and guinea pigs, it is suggested that the proptosis induced in fish has a genesis similar to that of the other species. Mast cells were hardly observed in Carassius auratus whenever the histological sections were exposed to aqueous solutions. With due precautions, however, an increase in mast cell number in the intestinal submucosa was noted.