NAGAHAMA, Y., W. C. CLARKE, and W. S. HOAR. 1977. Influence of salinity on ultrastructure of the secretory cells of the adenohypophyseal pars distalis in yearling coho salmon (Oncorhynchus kisutch). Can. J. Zool. 55: 183-198. Six different types of secretory cells were identified by light and electron microscopy in the adenohypophyseal pars distalis of yearling coho salmon acclimated to fresh or salt water. Prolactin cells are markedly more active in the freshwater than the seawater fish; these cells exhibit definite functional activity 3 days after transfer from salt to fresh water, indicating an osmoregulatory role of prolactin in the freshwater environment. Plasma sodium showed a significant decline 6h after transfer from sea water to fresh water and, even after I week, remained lower than in the fully acclimated freshwater fish. Corticotropic (ACTH) cells did not appear cytologically different in freshwater and seawater fish. GH cells, the most prominent cells in the proximal pars distalis, appear more numerous and more granulated in the seawater fish, suggesting an osmoregulatory involvement in young coho salmon. Putative thyrotropic (TSH) and putative gonadotropic cells (GTH) can be distinguished by differences in granulation; only one type of GTH cell is evident with ultrastructural features that differ from those of sexually mature salmon. Stellate, non-granulated ceUs occur in all regions of the adenohypophysis but more frequently in the prolactin follicles; they are much more prominent in the seawater than freshwater fish. NAGAHAMA, Y., W. C. CLARKE et W. S. HOAR. 1977. Influence of salinity on ultrastructure of the secretory cells of the adenohypophyseal pars distalis in yearling coho salmon (Oncorhynchus kisutch). Can. J. Zool. 55: 183-198. L'examen aux microscopes photonique et electronique de la pars distalis adenohypophysaire de saumons argent& d'un an acclimatts a I'eau douce ou B I'eau salee revele I'existance de six types distincts de cellules secretrices. Les cellules a prolactine sont beaucoup plus actives chez les saumons acclimates a I'eau douce; ces cellules manifestent une activite fonctionnelle bien definie 3 jours apres le transfer des poissons d'eau salee en eau douce, mettant ainsi en lumiire le r61e osmoregulateur de la prolactine en eau douce. Six heures aprks le passage d'eau salee en eau douce, on enregistre une diminution marquee du sodium plasmatique; m2me apres une semaine, ce taux reste plus bas que chez les poissons complttement acclimates al'eau douce. Les cellules a ACTH ne se distinguent pas cytologiquement chez les deux groupes de poissons. Les cellules BGH, les plus grosses de laportion proximale de la pars distalis, semblent plus nombreuses et plus granuleuses chez les poissons d'eau salee, ce qui peut indiquer qu'elles ont un r6le osmorkgulateur chez le ieune saumon amente. Les cellules a TSH et les cellules B GTH se distinguent par leurs granulations; un seil type de cellules B GTH comporte des ultrastructures distinctes de celles au'on trouve chez les saumons B maturi...