Succinate dehydrogenase (SDH) activities and cross-sectional areas (CSAs) of tibialis anterior motoneurons and muscle fibers were determined in 20-, 40-, and 60-week-old male senescence-accelerated mice (SAMP6), and compared with those in age-matched accelerated-senescence resistant mice (SAMR1). The mean CSA of motoneurons in SAMP6 decreased at 60 weeks, primarily due to a selective loss of large (> 400 μm2) motoneurons. The mean SDH activity of motoneurons with CSAs between 100 and 400 μm2decreased in SAMP6, but not SAMR1, at 60 weeks. The mean muscle fiber SDH activities and CSAs in SAMP6 decreased at 60 weeks. There were no differences in the mean SDH activity or CSA of motoneurons or muscle fibers among 20-, 40-, and 60-week-old SAMR1. These results demonstrate that mice which have been shown to have a variety of accelerated-senescent features also have an earlier onset of age-related changes in motoneurons and the muscle fibers that they innervate when compared with age-matched accelerated-senescence resistant mice.