Both in man and in animal models, changes in food intake and body composition in later life have been reported after alterations in perinatal nutrition. Therefore, we hypothesised that early postnatal undernutrition in the rat induces permanent changes in energy balance. Food restriction (FR) during lactation was achieved by enlarging litter size to twenty pups, whereas control animals were raised in litters containing ten pups. Energy intake and resting energy expenditure were determined in adult males. Early postnatal FR resulted in acute growth restriction followed by incomplete catch-up in body weight, body length and BMI. At the age of 12 months, middle-aged FR males had significantly lower absolute resting energy expenditure (200 v. 216 kJ/24 h, P = 0·009), absolute energy intake (281 v. 310 kJ/24 h, P = 0·001) and energy intake adjusted for BMI (284 v. 305 kJ/24 h, P = 0·016) than controls, whereas resting energy expenditure adjusted for BMI did not differ significantly between the groups (204 v. 211 kJ/24 h, P = 0·156). The amount of energy remaining for other functions was lower in FR males (80 v. 94 kJ/24 h, P = 0·044). Comparable data were obtained at the age of 6 months. These results indicate that in rats energy balance can be programmed by early nutrition. A low early postnatal food intake appears to programme these animals for a low energy intake and to remain slender in adult life.