We previously observed an increased SNS activity which was partly responsible for a defect in the insulin secretion response to glucose after post-weaning protein-energy restriction (PER) in female rats. These results, together with other data on low-protein feeding, suggested that a low protein-to-energy ratio (P/E) in the diet could stimulate energy expenditure (EE), but direct measurements of EE have never been reported under conditions of PER. The goal of the present study was thus to quantify the changes induced by PER to : body composition, the various parameters of EE and plasma triiodothyronine levels. PER induced severe growth retardation but the subcutaneous white and interscapular brown adipose tissues masses were preserved. Basal metabolism, meal-induced thermogenesis and triiodothyronine levels were increased but substrate utilization by the working muscles was unaffected. meal-induced thermogenesis was increased by spontaneous activity in PER rats only. These results suggest that rats adapt to a low P/E in the diet by burning part of their excess non-protein energy and storing the remaining excess in subcutaneous adipose tissue.