The effects of chronic (40%) food restriction from 6 weeks of age were studied in 28-month-old male Fischer-344 rats; the results were compared with ad libitum-fed, old and young male rats at 28 and 3 months of age, respectively. Pituitary luteinizing hormone was similar in all old rats and was significantly lower than in young rats. In old ad libitum-fed, but not in food-restricted rats, serum levels of LH, testosterone and T4 were significantly lower than in young rats. Serum levels of T3 did not differ between young and old rats. Type-II 5'-deiodinase activity in brown adipose tissue was similar in both groups of old animals and was significantly depressed as compared with that in young rats. Serum levels of triglycerides were significantly depressed in food-restricted rats, but were significantly increased in ad libitum-fed rats as compared with young rats. Both groups of old rats had significantly elevated serum levels of cholesterol over that in young rats, but the level was significantly lower in food-restricted as compared to ad libitum-fed animals. The results are consistent with the notion that life-long food restriction tends to preserve the activity of many metabolic functions.