We examined the effects of exogenous growth hormone (GH) treatment on the soleus and rectus femoris muscles of young female rats. Rat GH (1.8 IU/mg) was administered for 3 weeks by subcutaneous injection, twice a day, at doses of 0.5, 0.6, and 0.8 mg/day during the 1st, 2nd and 3rd week, respectively. Final body weight, as well as wet and dry weight, of the soleus and rectus femoris muscles were significantly greater in the GH-treated group, compared to controls. Muscle weight to body weight ratios did not differ between the two groups. The fiber type composition of the soleus muscle was determined by histochemical staining for myosin ATPase activity. No statistically significant difference was found between the GH-treated and the control groups in the percentages of fiber types. However, GH treatment significantly increased the cross-sectional area of type II fibers of the soleus muscle. These results suggest that, in young female rats, acceleration of body weight gain by homologous GH administration is accompanied by a proportional hypertrophy of skeletal muscle mass. Increased muscle mass is due to hypertrophy of muscle fibers. Type II muscle fibers appear to be more sensitive to GH stimulation.