Streptozotocin (STR), an agent known to induce damage of pancreatic B cells in mammals, was used to study changes of plasma insulin concentration and hepatic insulin-like growth factor I (IGF-I) mRNA levels in juvenile coho salmon, Oncorhynchus kisutch. Fish were injected intraperitoneally with either 200 mg/kg body wt STR or the vehicle. All fish fed and grew throughout the experiment; however, the STR-injected fish had lower instantaneous growth rates than control fish (1.4% and 2.1%, correspondingly). Plasma insulin concentration and hepatic IGF-I mRNA levels in STR-treated fish were significantly lower than in the control fish, whereas there was no difference between the two groups of fish in plasma concentrations of either glucagon or growth hormone. Impaired insulin production caused by STR injection was either coincident with or slightly preceded reduction in hepatic IGF-I mRNA expression, implying that insulin may affect IGF-I production and, consequently, fish growth.