Experiments were performed to investigate the effects of ovine growth hormone (oGH) on both the ultrastructural features of chloride cells and the ability of gills to extrude Na+ after transfer into seawater. February presmolts and June parrs of the Atlantic salmon (Salmo salar) were implanted with oGH. In such animals, spontaneously showing a poor ability to adapt themselves to seawater Life, GH significantly increased gill Na+-K+-adenosinetriphosphatase activity as well as gill sodium efflux into seawater. When examined by electron microscope, two types of chloride cells (alpha- and beta-types) were identified in control parrs and presmolts. GH treatment induced an increase in size and number of alpha-cells that displayed an extensive tubular system, while the beta-cells, thought to be specific to freshwater life, decreased in number. There was, concomitantly, an increase in number of accessory cells associated with the apical portion of the alpha-cells and, as a result, the formation of extensive shallow junctions between these cell types. Such functional and ultrastructural modifications that mimicked those naturally occurring during the last steps of the smoltification strongly suggest that GH stimulates the differentiation of freshwater chloride cells toward a seawater type.