The effects of high carbohydrate (CH) diets on circulating levels of insulin and glucagon were studied in rainbow trout (Oncorhynchus mykiss). Fish (76.87 +/- 8.11 g) were fed for 60 days with three isocaloric diets: control (C) (12% CH), W (28% CH mainly from wheat), and S (28% CH from wheat plus gelatinized starch). After the diet treatment, trout fed the enriched CH diets showed a higher hepatosomatic index and liver glycogen content than controls. In addition, plasma glucose levels were also higher but, on the contrary, circulating insulin levels were greater in the control group. After the period of diet adaptation, fish were sampled 3, 6, 9, and 24 h after food administration. Glycemia levels correlated with the dietary carbohydrate content, and were minimum in controls (from 96.02 +/- 3.77 to 118.97 +/- 6.08 mg/100 ml), followed by W group (from 124.60 +/- 7.46 to 172.19 +/- 11.36 mg/100 ml) and maximum in the S group (from 133.51 +/- 9.36 to 217.88 +/- 13.36 mg/100 ml). Postprandial glucagon profiles showed an inverse relationship with glycemia, indicating that the ingestion of glucose inhibits glucagon secretion. There were no significant differences in postprandial insulin levels between groups except for W group, which presented lower levels of circulating insulin 9 h after feeding. These results indicate that in trout fed a CH diet glucose affects the secretion of glucagon more than insulin during the postprandial period.