Carbohydrate-induced hypertriglyceridemia is easily produced in the rat, and fructose has been shown to be particularly potent in this regard. In this study we have compared the effects of feeding rats diets high (66% of total calories) in fructose or glucose on various aspects of carbohydrate and lipid metabolism. The results confirmed previous observations that fructose (456 ± 276 mg/dl) was more potent (p < 0.001) in raising plasma TG concentration than was glucose (242 ± 13 mg/dl), and indicated that the difference in magnitude of hypertriglyceridemia produced by the two carbohydrates was closely related to the ability of the test diets to increase VLDL-TG secretion (r = 0.85, p < 0.001). Both glucose and fructose feeding led to comparable degrees of hyperinsulinemia, and plasma TG concentrations increased before hyperinsulinemia evolved in fructose-fed rats. Therefore, it was concluded that fructose can act directly on the liver to increase VLDL-TG secretion, and that fructose-induced hypertriglyceridemia can occur in the absence of hyperinsulinemia. On the other hand, the rise in plasma TG concentration produced by fructose was reduced dramatically in exercise-trained rats, and this was associated with a decrease in plasma insulin concentration. Based upon these observations, we suggest that fructose feeding produces hypertriglyceridemia by directly stimulating hepatic VLDL-TG secretion, as well as by producing insulin resistance and hyperinsulinemia, and that it is the combined effect of these two separate actions which accounts for the magnitude of fructose-induced hypertriglyceridemia.