A morphometric study of disuse atrophy was done on the rat anterior tibilalis muscle over a 13-day period after immobilization of the hind leg by pinning the knee and ankle joints. Cross-sectional areas of individual muscle fibers were measured using image analysis, a new and precise technique. Three types of muscle fibers, light, medium and dark, were defined using a modified myosin-ATPase reaction on cryostat-cut sections. Each of the 3 fiber types was found to show a degree and time course of atrophy unique to itself. At the 0.01 confidence level, the dark fibers (with strongest ATPase reaction) did not shrink significantly. Both the light (ATPase negative) and medium fiber did show significant atrophy at that level; the light shrinking more than the medium. Simple visualization of the light microscopic images was often found to be misleading. Thus, the need was documented for a precise technique for analysis of fiber size distribution.