Adult male Wistar rats weighing 320 +/- 20 g at the beginning of the experiment were divided into two equal lots. A reference lot (T) was fed on a balanced diet containing 23,5 p. 100 mixed protein, for 60 days. A deficient lot (E) was fed on a low protein diet (3 p. 100 cereal protein) for 30 days (malnutrition), then on a balanced diet for 30 days (refeeding). When the food intake was expressed in 100 g of body weight/day, both lots ate about the same amount throughout the experiment. On the contrary, the amount of N ingested/100 mg of body weight/day by the E rats was only 11 p. 100 of that of the T lot. After 30 days of refeeding, the mean weight of the E rats equalled 90 p. 100 of that of the T rats. In the E lot, during malnutrition, the nitrogen balances were always positive and their nitrogen CDU were ranged between 73-83 p. 100 in comparison with 86-88 p. 100 in the T lot. The protein concentration of cardiac ventricles was the similar in both lots of rats, during malnutrition and refeeding periods. On the contrary, the myofibrillar protein concentration was increased in the E rats during these two phases. After separation of subunits sizes of muscle proteins by sodium dodecyl sulphate gel electrophoresis, we observed that the concentrations of myosin, actin, troponin-tropomyosin complex, myosin light chains, evolved in the same way that the total myofibrillar proteins. A relative degree of cardiac myocontractile protein sparing is suggested in our chronic protein malnutrition of adult rats.