After 10 or 30 d of growth arrest due to undernutrition during the early weaning period, rehabilitated rats fed ad libitum showed the phenomenon of catch-up growth, ie, gains in body weight and body protein were greater by 50% and 25%, respectively, (P < 0.001) than those of controls with similar starting body weight [ie, weight-matched (WM) controls]. These increases, however, were entirely dependent on the higher food intake because they failed to occur when food intakes were maintained at similar amounts to those for WM controls. In contrast, independent of whether the rehabilitated groups were spontaneously hyperphagic relative to WM controls or made normophagic by pair-feeding to WM controls, the gross energetic efficiency was increased (P < 0.01) and body fat gain was more elevated (2-2.5 fold, P < 0.001) during the first 2 wk of refeeding compared with WM controls--differences that were uninfluenced by the duration of growth arrest. Taken together, these studies suggest that the often reported impressive gains in body fat during recovery from malnutrition may result not only from unbalanced diets or excess dietary intake, but also from a transitory enhancement in the efficiency of food utilization and a shift in energy partitioning in favor of an acceleration for the replenishment of fat stores.