AbstractBenjamin, M.1980. The response of prolactin, ACTH, and growth hormone cells in the pituitary gland of the three‐spined stickleback,Gasterosteus aculeatusL. formleiurus, to increased environmental salinities. (Department of Anatomy, University College, Cardiff, Wales, U.K.) —Acta zool.(Stockh.) 61(1): 1–7.The time‐sequence of response of the prolactin, ACTH and growth hormone cells in the pituitary gland of theleiurusform of the three‐spined stickleback,Gasterosteus aculeatusL., to a transfer from freshwater to seawater, was studied by light microscopy. The appearance of the pituitary was compared with that of animals caught in brackish or sea water. The prolactin cells respond only slowly to seawater by cytological changes interpreted as signs of decreased secretory activity. It is thus suggested that prolactin is unlikely to be important for osmoregulation in this stickleback. The growth hormone cells are more active in all seawater‐adapted animals, while the ACTH cells are less active—although only in laboratory experiments. The differing responses of the pituitaries of theleiurusandtrachurusforms ofG. aculeatusandPungitius pungitiusto high salinities are compared. Even in species whose pituitaries are virtually identical, the response to high salinites may