Introduction From the standpoint of the relative activity of a number of enzymes in individual muscle fibers, the histochemical studies presented in a companion article1disclosed the presence of eight types of fibers in the calf muscles of the rat. The various types of fibers formed a continuum which could be arbitrarily divided into three groups. The gastrocnemius and plantaris were found to be composed of fibers of all the eight types. Within the gastrocnemius, the different types of fibers were found to have a definite topographical arrangement, some portions containing a majority of fibers of group I and other portions a predominance of fibers of group II. The soleus consisted entirely of fibers of group III. One of the aims of the presently reported combined histochemical and quantitative study was to obtain quantitative data on the activity of various enzymes in the three groups of fiber types. For