The pattern of fasting nutrient oxidation was measured in 11 healthy women of widely different body mass index by measurements of indirect calorimetry and urinary nitrogen excretion on 12 consecutive days. The women were given a diet of normal composition (44.5 per cent energy as carbohydrate; 40.7 per cent energy as fat) for 6 days and a high-carbohydrate diet (54.4 per cent energy as carbohydrate; 30.8 per cent energy as fat) for 6 days, in amounts designed to maintain energy balance for each woman. The differences in fasting respiratory quotient (RQ) between the women on the same diet were highly significant (P less than 0.001). The fasting RQ and the fasting carbohydrate and fat oxidation were significantly different between the two dietary periods, with the average difference in fasting RQ (0.027) being of similar magnitude to the difference in the food quotient (FQ) (0.026) between the two diets. The differences in fasting nutrient oxidation between individuals and between diets suggest differences in the extent of overnight carbohydrate storage as glycogen. There was no evidence that these differences were related to the tendency to gain weight in these subjects.