In Wistar rats a 10-hr swim caused an immediate increase of 3-methylhistidine and free tyrosine content in the white portion of quadriceps muscle. In the red portion the elevated levels of these amino acids were observed within 6 to 48 hrs of postexercise recovery. The glycogen depletion was substantially higher in the red portion of the muscle, indicating that the most active were oxidative-glycolytic fibres during the exercise. In conclusion, during exercise the most active muscle fibres do not contribute to the mobilization of protein resources. In a later part of postexercise recovery, catabolic changes occur in most active fibres, constituting, reasonably, a part of enhanced protein turnover.