Two surgical techniques were used to increase the work load per fiber in mouse skeletal muscles. These were, part removal of the muscle and incapacitation of a synergetic muscle. Both techniques were used for the soleus muscle. They resulted in the fiber size distribution of this muscle, which was normally unimodal, becoming distinctly bimodal. The same effect was obtained for the part removal of the anterior tibialis muscle. For the above mentioned muscles the extent of hypertrophy was very considerable. The recorded increase in the mean fiber cross-sectional area was in the region of 56% to 92%. Part removal of the biceps brachii however produced only a comparatively slight increase in fiber size (9% ). In this case the normal muscle was bimodal and the effect of part removal was to slightly increase the size of the second peak. The reason for the two peaks occurring in the distributions of fiber size in some muscles was due to the presence of fibers at a basic level of development and also the presence of hypertrophied fibers. The increase in mean fiber size resulting from an increased work load was in all cases, due to a proportion of the basic fibers undergoing hypertrophy and not due to a gradual increase in the size of all the fibers.