The immediate response of protein degradation to food intake and the factors for its regulation in rat skeletal muscle were examined. The concentration of N tau-methylhistidine (MeHis) in serum and the rates of MeHis release from isolated soleus and extensor digitorum longus muscles were reduced in the period from 3 to 6 h after refeeding, indicating that the rate of myofibrillar protein degradation in the rat decreased immediately after refeeding. Changes in the serum concentration of insulin and corticosterone were not synchronized with those in the myofibrillar protein degradation. When rats were fed on a protein-free diet, no reduction of serum MeHis concentration or of the rate of MeHis release from isolated muscles after refeeding was apparent. Furthermore, there was a tendency toward suppressing myofibrillar protein degradation with a higher protein content of the diet. These results suggest that the suppression of myofibrillar protein degradation by food intake was regulated by dietary proteins.