The kinetic mechanism of rabbit muscle glycogen synthase I was investigated by determining isotope-exchange rates at chemical equilibrium between uridine diphosphoglucose (UDPG) and glycogen and between UDPG and uridine 5'-diphosphate (UDP). The rates were followed simultaneously by use of UDPG labeled with 14C in the glucose moiety and with 3H in the uracil group. They were found to be independent of the concentrations of glycogen and the UDPG-UDP pair, averaging 6 X 10(-9) mol min-1 mg-1, with a ratio of UDPG-glycogen exchange to UDPG-UDP exchange of 0.85-0.95. The conclusion is that glycogen synthase has a rapid equilibrium random bi bi mechanism. The previously reported slow activation of glycogen-free synthase in the presence of glycogen was examined kinetically. The activation rate appears to be independent of glycogen concentration over a wide range, while the maximum activation is related to the third or fourth root of the glycogen concentration. This suggest that the slow bimolecular reaction mechanism proposed for human polymorphonuclear leucocyte glycogen synthase I [Sølling, H., & Esmann, V. (1977) Eur. J. Biochem. 81, 129] does not apply to rabbit muscle synthase I. The rate of exchange of glycogen molecules in the complex between glycogen and rabbit muscle synthase I under conditions where the enzyme is catalytically active was estimated by a novel method. The enzyme-glycogen complex was treated with [glucose-14C]UDPG and glycogen of different molecular weight. The distribution of isotope between the two forms of glycogen was determined after their separation by agarose gel chromatography. A rate constant of 0.3 min-1 was estimated for the exchange. It can be calculated, on the basis of the specific activity of the enzyme (20 mumol min-1 mg-1) and its action pattern, that hundreds of individual chains in the glycogen molecule must be available to the enzyme during the average lifetime of the complex. A mechanism is proposed for this process.