Six normal-weight subjects were fed five separate diets [isocaloric--20, 40 and 80 per cent carbohydrate (CHO) and hypercaloric 20 and 40 per cent CHO] for 5 days each. Intravenous glucose-tolerance tests were performed on on Day 6. Insulin responses were positively related to caloric and CHO content of the diets, both of which affected insulin secretion independently of each other. Glucose disappearance rates were highly correlated (r = 0.80) with the acute insulin response to glucose. Fasting plasma-glucose levels (FPG) were also affected by both calories and CHO content, but in a more complex manner. Raising CHO content in isocaloric diets or increasing calories in diets of normal CHO content increased FPG. However, if either calories or CHO content were high, increasing the other had no further effect on FPG. Thus, there was an interaction between calories and CHO on FPG. In 18 instances, the diet was continued on Day 6 and another intravenous glucose tolerance test was performed the following morning, 90--120 minutes after 30mg of propantheline bromide by mouth. Glucose disappearance rates were increased in 16 of the 18 paired tests (P less than 0.005) after the anticholinergic agent. There was no effect on FPG or the insulin response to glucose. CONCLUSIONS both calories and CHO content are positively related to insulin secretion after 5 days of dietary manipulation. Glucose disappearance rates are also influenced, probably via the effect on insulin secretion. Both increased calories and CHO content will raise FPG; but each will have this effect only in the presence of a normal intake of the other. Thus, unless possible interactions between calories and CHO are taken into consideration in the design and interpretation of future dietary studies, erroneous conclusions may be reached. Finally, cholinergic blockade will enhance the disposal of intravenous glucose in the absence of any effect on peripheral insulin levels.