Spinal isolation (SI), i.e., the isolation of the lumbar spinal cord via a rostral and a caudal cord transection and bilateral dorsal rhizotomy, was used to determine the effects of chronic (6 months) inactivity on the size and metabolic properties of fibers in the cat soleus. Fibers were classified as dark or light, based on their staining reactions to myosin ATPase, alkaline preincubation, and immunohistochemically as expressing fast and/or slow myosin heavy chains (MHC). Succinate dehydrogenase (SDH) and alpha-glycerophosphate dehydrogenase (GPD) activities were assessed histochemically. Following SI, both the light and the dark ATPase fibers in the SI cats were significantly smaller than the light ATPase fibers in the controls. Normally 100% of the fibers were light ATPase and reacted exclusively with the slow antibody. After SI, approximately 45% of the fibers were dark ATPase fibers, many reacting with both fast and slow MHC antibodies. The total amount and concentration of GPD were higher in the light and dark ATPase fibers in SI compared with light ATPase fibers in controls. In contrast, although the total amount of SDH per fiber was decreased, reflecting the decrease in fiber size, the mean SDH concentration per fiber was unchanged following SI. These data indicate that there is a close coordination in the regulation of GPD activity and the type of myosin. SDH activity, on the other hand, appears to be resistant to decreased levels of activity and unloading, i.e., there seems to be a minimum level of oxidative potential in the soleus that is independent of activity level. Fiber sizes, however, are very sensitive to less-than-normal amounts of neuromuscular activity and/or loading.