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A creatine analogue, beta-guanidinopropionic acid (beta-GPA), was administered in the food (2% wt/wt) and the water (0.5% wt/vol) of male CD-1 mice. Uptake of the phosphorylated analogue and depletion of phosphocreatine in hindlimb muscle was monitored by 31P nuclear magnetic resonance and was found to be complete within 7 wk. After this time, the isomyosin composition of soleus, extensor digitorum longus (EDL), and ventricle was analyzed by pyrophosphate gel electrophoresis. The analogue was found to induce significant alterations in the type of myosin expressed in soleus and EDL. Normal soleus contains both intermediate (IM) and slow (SM) myosins, and treatment reduced the relative content of IM by approximately 50%. In EDL, treatment decreased fast isomyosin FM3 by 60% compared with controls. Sodium dodecyl sulfate-gel electrophoresis also showed a decrease of parvalbumin in EDL by approximately 50%. Treatment had no significant effect on the isomyosin composition of heart ventricle. Levels of physical activity and concentrations of serum glucose and thyroxine of treated mice were not significantly different from controls. These results indicate a role for intracellular energetics in mediating adaptive changes in the phenotype of muscle in mature animals.