The low dietary starch utilisation by rainbow trout (Oncorhynchus mykiss) may be attributed to a dysfunction of the nutritional regulation of the hepatic glucose/glucose-6-phosphate cycle. The present study was initiated to analyse the regulation of activity and gene expression of hepatic glucokinase (GK) and glucose-6-phosphatase (G6Pase) by dietary carbohydrates in this species. We found that even a single meal containing 24% of glucose is sufficient to induce the GK expression (mRNA and activity) as in mammals. In contrast, although the inhibitory effect of dietary glucose on G6Pase expression is observed at the molecular level, the G6Pase activity is not significantly inhibited by dietary glucose. Thus, in contrast to the gluconeogenic G6Pase enzyme, a rapid adaptation of the hepatic glycolytic GK enzyme to dietary glucose seems effective in rainbow trout. These results suggest that in carnivorous rainbow trout, the liver is capable to strongly regulate the utilisation of glucose but not the synthesis of glucose.