Age-related changes in liver protein synthesis of ad libitum fed and food-restricted SPF Fischer 344 rats have been measured. In ad libitum fed animals, valine incorporation increased from 3 to 6 mo of age, decreased to the 3-mo level at 12 mo, remained relatively constant through 18 mo, and then declined further at 24 mo of age. By 24 mo valine incorporation was 30% of the 6-mo value. Food restriction had no effect at 3 mo of age (6 wk of restriction) but at 6 mo the rate of valine incorporation was 35% greater than the control, an increment that was maintained throughout the life of the animal. Therefore, although food restriction does not prevent the age-related decline in protein synthesis, it does maintain the rate of liver valine incorporation at levels greater than those observed in the liver of the ad libitum fed animal.