In both breast and leg muscle from 6-, 9-, and 12-month-old chickens, held for aging at 0°C, the buffer-extractable nitrogen rapidly decreased after death during the onset of rigor and gradually increased to a maximum value during post-rigor aging. Changes in extractable nitrogen occurred mainly as a result of changes in the solubility of myofibrillar proteins. Changes in sarcoplasmic and stroma protein fractions were small. In the nonprotein-nitrogen fraction, some of the amino-acid-containing polymers were removed during the onset of rigor mortis as a result of their interaction or aggregation with proteins. During the post-rigor tenderization period, amino acids and peptide increased in meat as a result of proteolysis. Different rates of post-mortem tenderization in breast and leg muscle appear to be related to the differences in stroma protein content of muscle. Proteolysis seems to weaken or break the bonds which bind myofibrils to the matrix of the muscle and causes protein changes that are responsible for post-rigor tenderization.