The elaboration and distribution of newly formed proteins in the striated muscle of 21-day-old mice were investigated by quantitative radioautography at intervals between 2 and 240 min after intravenous injection of tritiated leucine. In radioautographs, the localization and the relative label concentration were comparatively estimated for the different components of mitochondria-rich fibres, in particular of red fibres, from the tibialis anterior muscle and of mitochondria-poor fibres from the oesophageal muscle. As early as 2 min after injection, radioactivity was detected over the nucleus, the polysome-rich sarcoplasm, the A and I bands, the Z lines, and the mitochondria in the two fibre types. Label localization did not change with time. The relative label concentration increased similarly in the polysome-rich sarcoplasm and the A and I bands of both fibre types within 30 min after injection, a confirmation that biosynthesis of myofibrillar proteins takes place rapidly. In each case, concentration was higher in the Z lines than in the I bands, and higher in the I bands than in the A bands, thus showing "in vivo" that the rates of synthesis of sarcomere protein components are not uniform. However, the relative label concentration was found to be higher in the Z lines of mitochondria-poor fibres than of mitochondria-rich fibres: this suggests that a higher synthetic rate of Z line protein, and probably of alpha actinin, is characteristic of the first type. Inversely, the concentration was higher in the mitochondria of mitochondria-rich fibres. This lead to the belief that high rate of protein synthesis in these organelles may account for the high rate of label incorporation into this type of fibre.