The mechanical properties of individual motor units in the medial gastrocnemius muscle, as well as the whole muscle properties and innervating motor nucleus, were investigated in food-restricted, male Fischer 344/DuCrj rats at ages of 4, 7, 12, 21/22, 27, 31 and 36 months. The tetanic tension of the type S units continuously increased until the age of 36 months. Those of type FF and FR units declined from 21/22 months to 27 months of age, but did not change further while the whole muscle tension decreased greatly. The atrophy of muscle fibers, the decline in motoneuron number and axonal conduction velocity, and the decrease in the post-tetanic potentiation of twitch contraction of motor units seemed to start after 21/22 months of age and were accelerated with advancing age. Prolongation of twitch contraction time was evident for only type S and FR units in 36-month-old rats. The fatigue index was greatly increased for type FF units in 36-month-old rats. These findings indicated that the progress of changes in various properties occurring in the senescent muscle was different in terms of their time course and degree, and also dependent on the types of motor unit. The atrophy and decrease in specific tension of muscle fibers affected the decline in tension output of motor units. This was effectively compensated for by the capture of denervated muscle fibers over time.