In this study, we set out to examine the role of the somatotropic axis in the ion-regulation process in rainbow trout. Specifically, our objective was to examine whether plasma insulin-like growth factor-binding proteins (IGFBPs) are modulated by gradual salinity exposure. To this end, freshwater (FW)-adapted rainbow trout were subjected to gradual salinity increases, up to 66% seawater, over a period of 5 days. During this acclimation process, minimal elevations in plasma Ca2+ and Cl- were seen in the salinity-acclimated groups compared with FW controls. There were no changes in plasma Na+ levels, and only a minor transient change in plasma cortisol levels was seen with salinity exposure. The salinity challenged animals responded with elevations in plasma growth hormone (GH) and IGF-I levels and gill Na+-K+-ATPase activity. We identified IGFBPs of 21, 32, 42, and 50 kDa in size in the plasma of these animals, and they were consistently higher with salinity. Despite the overall increase in IGFBPs with salinity, transient changes in individual BPs over the 5-day period were noted in the FW and salinity-exposed fish. Specifically, the transient changes in plasma levels of the 21-, 42-, and 50-kDa IGFBPs were different between the FW and salinity groups, while the 32-kDa IGFBP showed a similar trend (increases with sampling time) in both groups. Considered together, the elevated plasma IGFBPs suggest a key role for these binding proteins in the regulation of IGF-I during salinity acclimation in salmonids.