To see if both the amount and source of carbohydrate consumed determined postprandial glucose and insulin responses of mixed meals, eight nondiabetic subjects took five different mixed meals containing variable energy (1650-2550 kJ), fat (8-24 g), protein (12-25 g) carbohydrate (38-104 g) and glycemic index (43-99). Incremental glucose and insulin responses for the five meals varied over a 2.3-fold range. Amount of carbohydrate alone was not significantly related to the mean glucose and insulin responses. However, using previously derived equations, amount of carbohydrate and glycemic index explained approximately 90% of the variability of the observed mean glucose and insulin responses (P = 0.01). We conclude that both amount and source of carbohydrate determine the glucose and insulin responses of lean, young, nondiabetic subjects after different mixed meals with variable glycemic index. Variation in protein and fat intake, over the range tested here, appears to have a negligible effect on postprandial glucose and insulin.