In this study we report that the use of a heterologous radioimmunoassay (RIA) is valid for the detection of insulin-like growth factor-I (IGF-I) levels in plasma of a variety of fish species. Parallelism between standard curves and plasma dilutions were observed and the standard curve obtained with mammalian IGF-I presented the same characteristics as that obtained with coho salmon recombinant IGF-I. The RIA was biologically validated since total plasma IGF-I values were significantly modified by different experimental conditions. Hyperinsulinemia induced either by arginine or insulin injection was accompanied by increases in IGF-I plasma levels in brown trout (Salmo trutta). In contrast, parallel decreases in insulin and IGF-I circulating levels were observed after 45 days of fasting and 20 days after a single streptozotocin injection. Administration of arginine in fasted fish led to a relative increase in insulin and IGF-I plasma concentrations, while arginine injection in fish previously treated with streptozotocin increased IGF-I levels only. The above data suggest that insulin, together with other factors, may act to increase the levels of IGF-I in plasma.