In an analysis of the gastrocnemius muscle, the microsomal fraction showed the highest cholinesterase (ChE) activity. The ChE activity of all fractions decreased to a greater extent after strong nerve crushing than after weak crushing. This change in the activity in the microsomal fraction was the most marked change observed. Although in the analysis of the soleus muscle the ChE activity was measured only in the homogenate and in the microsomal fraction, the results were the same as those obtained with the gastrocnemius. A preparation of vitamins B1, B2, and B12 (B complex) had little effect on the ChE activity in the gastrocnemius muscle. In the soleus muscle on the lesion side, the B complex increased the ChE activity to some extent after nerve crushing, but such was not significant. However, the B complex signifiicantly increased this activity in the soleus on the intact side. In the soleus muscle, strong nerve crushing induced more marked muscle atrophy than weak crushing. On the other hand, no significant difference was found in the gastrocnemius. Effects of the B complex on muscle atrophy were found in the soleus, but not in the gastrocnemius. These results suggest that the ChE activity in the microsomal fraction containing sarcoplasmic reticulum reflects the nervous disorders clearly, and that the B complex increases the ChE activity and muscle weight in the soleus, but not in the gastrocnemius muscle.