AbstractHypoxanthine‐guanine phosphoribosyltransferase (HGPRT) activity was measured in red cells and in skeletal muscles of normal and Duchenne subjects. [8‐14C] hypoxanthine was used as substrate, and 5‐phospho‐α‐D‐ribose 1‐diphosphate (PRPP) was used as the ribose‐5‐phosphate donor. The [8‐14C] inosine monophosphate (IMP) formed was separated by high‐voltage electrophoresis, and radioactivity was measured by liquid scintillation counting. HGPRT activity in Duchenne and normal red‐cell hemolysates was similar, but such activity was significantly higher in Duchenne than in normal muscle homogenates. Red cells of both normal and Duchenne subjects had significantly higher enzyme activity than did skeletal muscles. It is suggested that increased HGPRT activity may be involved in enhancing protein synthesis by increasing intracellular levels o