Diet composition and energy content modulate free fatty acid (FFA) release. The aim of this study was to evaluate the dose–response effects of euenergetic variations in dietary carbohydrate and fat content on postabsorptive FFA release. The rate of appearance (Ra) of palmitate was measured by infusion of [2,2-2H2]palmitate after an overnight fast in six healthy men on three separate occasions, i.e. after 7 d on euenergetic control, high-carbohydrate and high-fat diets. The protein content and composition was identical for each diet. Postabsorptive plasma fatty acid concentrations were not different between the high-carbohydrate and control diets (0·36 (SE 0·07) V. 0·43 (se 0·04) mmol/l), but were increased after the high-fat diet (0·75 (se 0·09) mmol/l, (P<0·01 compared with the other diets). Ra palmitate was not different between the high-carbohydrate and control diets (1·36 (se 0·20) v. 1·47 (se 0·15) μmol/kg per min). However, Ra palmitate was increased to 2·36 (se 0·26) μmol/kg per min after the high-fat diet (P<0·01 compared with the other diets). The fatty acid flux and whole-body fat oxidation were not affected by the high-carbohydrate diet compared with the control diet, but were increased by 67 and 47 % respectively, on the high-fat diet (P<0·01 compared with the other diets). A euenergetic high-fat diet results in increased postabsorptive FFA release and fat oxidation, whereas a euenergetic high-carbohydrate diet does not affect these variables of fat metabolism.