The initial response of the IGF-I system and the expression and cellular localization of IGF type I receptor (IGF-IR) were studied in the gill of a euryhaline teleost during salinity acclimation. Exposure of striped bass (Morone saxatilis) to hyperosmotic and hypoosmotic challenges induced small, transitory (< 24 hours) deflections in hydromineral balance. Transfer from fresh water (FW) to seawater (SW) induced an initial decrease in plasma IGF-I levels after 24 hours in both fed and fasted fish. There was an overall decrease in liver IGF-I mRNA after SW transfer suggesting that decreased plasma levels may be due to a decline in hepatic IGF-I synthesis. No changes were observed in gill IGF-I mRNA, but SW transfer induced an increase in gill IGF-IR mRNA after 24 hours. Transfer from SW to FW induced an increase in plasma IGF-I levels in fasted fish. In fed fish, no significant changes were observed in either plasma IGF-I, liver or gill IGF-I mRNA, or gill IGF-IR mRNA levels. In a separate experiment, FW acclimated fish were injected with saline or IGF-I prior to a 24 hours SW challenge. Rapid regain of osmotic balance following SW transfer was hindered by IGF-I. Immunohistochemistry revealed for the first time in teleosts that IGF-IR and Na⁺,K⁺-ATPase are localized in putative chloride cells at the base of the lamellae, identifying these cells in the gill as a target for IGF-I and IGF-II. Overall the data suggest a hyperosmoregulatory role of IGF-I in this species.