James B. Peter,* R. James Barnard, V. Reggie Edgerton, Cynthia A. Gillespie, and Kerstin E. Stempel abstract: Certain guinea pig and rabbit skeletal muscles, which are composed solely or predominantly of a single type of fiber as defined histochemically, were analyzed for various enzymatic and substrate characteristics. The results show that fiber types in the adult guinea pig and rabbit limb muscles can be conveniently classified into three categories on the basis of three criteria,(1) the contraction time of the fiber relative to others within the same animal,(2) glycolytic capac-ity, and (3) oxidative capacity. The rabbit semimembranosus accessorius and the white portion of the guinea pig vastus lateralis are fast twitch and have a very high anaerobic capac-ity, as indicated by glycogen concentration and phosphoryl-ase, lactate dehydrogenase and mitochondrial a-glycerophosphate dehydrogenase activities. Cytochrome concentra-tion and succinate dehydrogenase activity are low, indicating a low aerobic capacity. Consequently these fibers, formerly termed fast-twitch white, are more explicitly called fast-twitch-glycolytic fibers. The red portion of the guinea pig vastus lateralis has a high glycogen concentration, moderate lactate dehydrogenase, and high phosphorylase and a-glycerophosphate dehydrogenase activities. The red vastus lat-eralis has the highest cytochrome concentration and succinate dehydrogenase activity, indicating a high aerobic capacity in