The effect of 12 weeks' light-moderate underfeeding on the 24-h energy expenditure of a fixed physical activity programme was studied in six male and six female subjects. During a control period of 2 weeks, measurements were made of 24-h energy expenditure by direct calorimetry, the lean and fat body masses and the subjects' habitual energy intakes. The average body weight was 107 per cent of ideal, and did not change significantly during the control period. The subjects were asked to continue to record their daily food intake while reducing it by 25 per cent and to continue their daily activity habits during a period of 12 weeks. The mean change in body weight was -2.9 kg with an average change in body energy content of -66 MJ. The 24-h energy expenditure was measured three times during the period, ie, after 4, 8 and 12 weeks from the start of the decreased energy intake, and found to be practically the same as the control value, ie, with average differences (s.d.) of -0.2(3.9), -0.8(5.7) and +0.6(4.2) per cent from the control value. In the six subjects who had the largest loss of body weight, this was found to be 5.0 kg, and the mean differences (s.d.) in energy expenditure were found to be -1.0(3.4), 0.0(5.6) and -0.6(3.9) per cent, respectively, from the control value. A significant part of the between-subjects variation in the differences from the control value is due to random measurement error and it is concluded that a loss of body energy content of the magnitude reported here, and due mainly to a reduced food intake, does not result in a significant increase in the efficiency of energy conversion, ie, in metabolic adaptation.