During the parr-to-smolt transformation (smoltification) of juvenile salmonids, preadaptive changes in osmoregulatory and ionoregulatory ability are regulated in part by the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis. If food intake is sufficient, plasma IGF-I increases during smoltification. On the other hand, plasma IGF-I typically decreases in fasting fish and other vertebrate animals. Because food availability is limited for juvenile salmonids undertaking an extended 6- to 12-week spring migration to and through the Snake-Columbia River hydropower system (northwestern USA), IGF-I concentrations might be expected to decrease, potentially compromising seawater tolerance. To address this possibility, yearling chinook salmon Oncorhynchus tshawytscha reared in three Snake River Basin hatcheries were sampled before release and at two downstream dams. Dry masses of migrating fish either did not increase during the migration (in 2000, an average-flow year), or decreased significantly (in 2001, a low-flow year). In both years, plasma IGF-I levels were significantly higher (1.6-fold in 2000, 3.7-fold in 2001) for fish sampled at the last dam on the lower Columbia River than for fish sampled prior to release. Plasma IGF-I concentrations in migrating fish may, nonetheless, have been nutritionally down-regulated to some degree, because plasma IGF-I concentrations in juvenile chinook salmon captured at a Snake River dam and transported to the laboratory increased in fed groups, but decreased in unfed groups. The ability of migrating smolts to maintain relatively elevated IGF-I levels despite restricted food intake and loss of body mass is likely related to smoltification-associated changes in hormonal balance. ?? 2004 Kluwer Academic Publishers.