Abstract
1. Growth hormone reduced the loss of weight or induced net growth of denervated soleus and plantaris muscles in hypophysectomized rats.2. Growth hormone did not alter the weight of the denervated muscle relative to its contralateral control, i.e. the hormone increased the sizes of denervated and control muscles proportionately.3. Similarly treatment with the hormone appeared to cause proportional growth in muscles undergoing work-induced hypertrophy and their contralateral controls.4. It is suggested that muscle size is determined by at least two independent factors, growth hormone and muscular work. Growth hormone appears to act as a biochemical ;amplifying system' for the cell's anabolic machinery and determines the absolute changes in muscle size which result from changes in muscular work.