Abstract
Histochemical myofibrillar adenosine triphosphatase (ATPase) reaction has been studied in the soleus muscles of males, castrate males and females of two different stocks of mice. Males were shown to have a significantly lower percentage of fibers with an alkali-labile ATPase in the soleus than either castrate males or females. The castrate males and females did not differ significantly from each other. This suggests that androgens can influence fiber type ratios. The two stocks of mice differ significantly with respect to the proportion of fibers with an alkali-labile ATPase in the soleus muscle. This is interpreted as a manifestation of genetic differences between the stocks.