This paper describes the glycation of myofibrillar proteins and its relationship with aging. The identification of N(ε)-fructoselysine in myofibrillar protein was done by the HPLC procedure and mass spectrometry. The N(ε)-fructoselysine content in mouse myofibrils during the experimental period of 59 weeks rose slightly from 0.56 to 0.74nmol/mg of protein. That in mouse actomyosin was higher and increased exponentially from 2.32 to 4.98 nmol/mg of protein during a period of 27 weeks, indicating a relationship between the glycation of mouse actomyosin and age. In the case of rats, although the N(ε)-fructoselysine content of myofibrillar proteins did not show a clear change like that of mice, the content in actomyosin gradually increased with age. Taking into account the decrease in muscle protein turnover with age that has been described in previous papers, these results imply a relationship between the glycation of myofibrillar protein and age. This is the first report on the glycation of muscle protein.