Recently, we have demonstrated in rainbow trout that environmental temperature may, independently of nutritional status, directly stimulate plasma growth hormone (GH) that is recognised as being an insulin-like growth factor (IGF) system regulator. The aim of this study was to determine whether temperature may directly regulate the IGF system or indirectly regulate it through plasma GH or nutritional status. For this purpose, rainbow trout were reared at 8, 12, or 16 °C and fed either ad libitum (similar nutritional status) to evidence the global effect of temperature, or with the same ration (1.2% body weight/day), to determine the temperature effect in fish with the same growth rate. Endocrine and autocrine/paracrine regulations of the IGF system were determined by measuring plasma IGF1 and IGF2, liver and muscle IGF1 and IGF2 mRNA as well as IGFRIa, IGFRIb mRNA, and the quantity of IGF type I receptor in muscle. Our results show that neither rearing temperature nor the nutritional status of fish affected the expression of both IGF receptor genes in muscle. Nevertheless, the quantity of IGF type I receptor determined by a binding study, appeared to be inversely proportional (P<0.05) to the rearing temperature without any relationship with nutritional status, suggesting a direct effect of temperature on its turnover. After 2 weeks of treatment, the levels of IGF1 mRNA in muscle at 8 °C were 2-fold higher (P<0.05) than at 16 °C in both ad libitum and restricted feed fish, whereas after 6 weeks, this difference was no longer observed. In both experiments, the levels of plasma IGF2 were 10-fold higher than the levels of plasma IGF1 (mean 105 ± 3.0 versus 13.5 ± 0.6 ng/ml), and plasma levels were correlated with their respective mRNA liver concentrations (r2=0.14 and 0.25, respectively; P<0.01). In the ad libitum feeding experiment, plasma and mRNA levels of IGF1 were related to the rearing temperature (P<0.05), while for IGF2 no effect was seen. In contrast, in the restricted feeding experiment, plasma and IGF2 mRNA levels were inversely proportional to the rearing temperature (P<0.0001) while plasma IGF1 was unaltered. Levels of plasma IGF1 were related to the growth rate in both experiments, while levels of plasma IGF2 appeared to be associated with the nutritional status of the fish. Our results suggest that the autocrine/paracrine expression of IGF1 and IGF2 in muscle is not a key regulator of the growth promoting effect of temperature. Conversely, temperature seems to promote growth through IGF1 secretion by the liver following GH stimulation, and impairment of nutritional status would prevent the IGF1 stimulation by temperature. In addition, the growth-promoting effect of temperature did not affect plasma IGF2, which appeared to be more related to the metabolic status of the fish.