The existence and quantitative contribution of an adaptive increase in whole-body metabolic efficiency in response to low caloric intake was examined after prolonged underfeeding in the rat. 24h energy expenditures (EE) and body composition were measured in chronically underfed rats which had been maintaining a stable body weight for several months after an initial period of substantial weight loss. Comparisons were made with ad libitum fed controls. The mean coefficient of variation for daily EE, assessed over three consecutive days, was 3% (range 1.7-4.0%) in the control group and 1.6% (range 0.7-2.1%) in the chronically underfed group, and hence reflect steady state conditions. Body composition was determined by chemical analysis, and as a proportion of body weight (BW), the fat content in the underfed group was reduced by 40% whereas lean body mass was significantly higher by 8%. Independently of the criteria utilized for metabolically active mass, 24h EE was lower in the underfed group than in controls, by 17%, 14% and 8% as a function of BW0.66, BW0.75 and lean body mass respectively (P < 0.01). Underfeeding was also associated with a substantial reduction in the level of external work, but in ad libitum fed animals similar reductions in external work (induced by limiting locomotion) failed to alter 24h EE significantly. Taken together, these studies provide evidence for an increased metabolic efficiency in response to low calorie intake well beyond the period of dynamic weight loss.