Morphological changes of the chloride cells and the α_1b subunit gene expression of Na⁺-K⁺-ATPase in triploid rainbow trout (70.6 g average weight) were studied upon direct transferring to 6, 12 and 18 ppt salinities. Changes in abundance, distribution pattern, and the sectioned area of the chloride cells was studied through classic histology and Na⁺ K⁺-ATPase localization was performed through immunofluorescence light microscopy using a mouse monoclonal antibody IgGα5. Gene expression of Na⁺-K⁺-ATPase α_1b subunit was studied by semi-quantitative gene expression methods.No mortality occurred among the fish in all salinities during the 10-days experimental period and treated fish kept their plasma osmolality at standard physiologic levels. All the fish also showed similar distribution pattern in their chloride cells that were distributed on filaments, between and over lamella. Histological studies confirmed some abnormal morphological changes such as lamella interruption. Immunohistochemical studies showed the highest number of the chloride cells on lamella and between lamella in 18 ppt and the maximum sectional area of the chloride cells in freshwater. Gene expression of Na⁺-K⁺-ATPase α_1b subunit had direct correlation with increasing trend of salinity. In conclusions, triploid rainbow trout was found to be adaptable to the various experimented salinities and could be recommended for rearing in brackish water.