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The biosynthesis of glycogen from UDPG 5 with a livcr enzyme has been reported previously ( l ). The reaction has been investigatcd further using a partially purified preparation form rat muscle with which the •• general properties of thc system have been studied. l\ f ETHODS AnalyticnlCl ycoge n wa s es timated by lhe phe.;ol-sulfuric acid method (2), after digest.i.D.Jl with KOH and ethlanol precipilat ion (3) . A sampJ.e of glywgrn prepared as described by Somogyi (4) wa s used a:; standard. lts co ncentral ion was checked against gl ucose using the k l nthrone method (5). U DP was estima ted as describcd by Cabib and L eloir (6) , but h alvwg the amounts o[ reagcnls. UDPG W\lS measured spectrophotometrica ll y with a partially purifiecl UDPG dehydrogenase (7). Phosphory lase was cstimratecl as dt scribed by Cori el al. (8). Protein was measurcd by the m ethods of Kunitz a ncl \1 cDonalcl (9) and of W arb urg ancl Christian ( lO). Amylase ac tivity was d e term ined under the ~a m e co nditions as the glycogen-formil.g enzyme but withoul U DPG or G-6-P. After clepn:!c inization with Ba (OH) , a nd ZnSO,, lh c reclucing substances were lltC'ast wcd according to Park and Johnson (11). G-6-P was cstim a ted spectrophotometricall y (12) . Radioacti,•ity was meas urecl with a gas-Elow co unter. Radioactive sugars in paper chrolll'atogra ms were locatecl wilh a silver reagent (13) and th en eluted, platee! , ancl co unted. App roximately halE oE thc added counts werc dctcctecl afte r this treatment.