Postnatal changes in the masseter muscle were investigated in the toothless (B6C3Fe-a/a-op/op) mouse with less-developed periodontal ligaments. From the start to the end of the weaning period, no distinct difference in histochemical staining of succinate dehydrogenase (SDH) activitiy in muscle fibers was detectable between op/op and normal (B6C3Fe-at/a-+/?) mice. Thereafter, differences in the SDH activity of muscle fibers between op/op and +/? mice gradually appeared in the masseter muscles and, by the third postnatal month, severely atrophied muscle fibers were observed in the superficial region of the masseter muscle of op/op mice, which was composed of white and intermediate muscle fibers. However, the nature of progression and the extent of atrophy of muscle fibers in the toothless mouse differed among regions of the masseter muscle. Moreover, the decrease in diameter of muscle fibers in the op/op mouse was larger than that in the +/? mouse fed a granulated diet. These observations suggest that the inevitable decrease in the sensory input due to less-developed periodontal ligaments may be involved in the the atrophy of masseter muscle fibers in the toothless mouse._