|In teleost fish, gill plays an important role of controlling ion transport mechanisms for the maintenance of ion- and osmoregulatory homeostasis in both fresh water and seawater. Leptin mRNA expression has been found in several fish species, and was also found in gills of Arctic charr (Salvelius alpinus) in this study. However, no study has been reported on the function of leptin in the gills of fish. The question arise f Leptin may have some function related to ion- and osmoregulation in fish gills. In this study, Arctic charr were sampled during their smolting period in freshwater. Seawater tolerance, as well as gill Na+, K+-ATPase activity of the fish increased from winter to spring in these fish, showing that they smoltified. There was no change in gill leptin mRNA expression during and after smolting in the fish acclimated to freshwater and hence, the results gave no evidence for a paracrine role of leptin in the regulation of the adaptational changes in gill during smolting. It can not be excluded, however, that leptin may be involved in the changes taking place in the gill in association with seawater exposure, and future studies must include sampling of gill tissue, and analyses of leptin mRNA expression after seawater transference.