Skeletal muscle hypertrophy has been investigated in several models including denervation, increased work induced by tenotomy of a synergist, and the addition of weights to the wing of a chicken. The stimulus for growth probably involves passive stretch along with increased tension development induced by the stretch reflex in innervated muscles. In early studies of hypertrophy of rat soleus and plantaris muscles, turnover measurements were interpreted as indicating increased protein synthesis and decreased degradation as the mechanism for the hypertrophy. However, these conclusions are probably unjustified. In particular it has been shown that there is a marked increase in reutilization and isotope recycling in muscle during hypertrophy. Several studies indicate that both protein synthesis and degradation are increased during hypertrophy. This has been shown in the transient hypertrophy of the denervated hemidiaphragm. During hypertrophy of the innervated anterior latissimus dorsi of chickens, there are increases in protein synthesis and degradation to the extent that as much as two-thirds of the total increase in synthesis may be associated with increased turnover or "wastage" and only one-third with growth.