The effects of an 8-wk exercise program were studied on growth of rats following a period of normal nutrition or undernutrition. Male Wistar weanling rats, underfed by fasting on alternate days for 4 wk, gained 56% as much weight as ad libitum-fed rats (carcass: 47% as much fat; 63% as much lean mass), and had markedly smaller fat depots, visceral organs, brains and hindlimb muscles and bones. Rats in both groups were then allowed free access to feed and subdivided into exercise or non-exercise groups. Eight wk of treadmill exercise (speed, 24 m/min; duration, 75 min/d; frequency, 5 d/sk) reduced weight gain in both normal-fed and underfed rats, but had a greater effect on underfed rats. Underfed rats that were exercised gained 76% as much weight (carcass: 54% as much fat; 92% as much lean mass), and tended to have lighter tissues than underfed rats that were not exercised. Data suggest that treadmill exercise following a postweaning period of undernutrition reduces "catch-up" growth in rats.