A lot of physiological activities are expected from glucose-1-phosp hate (G-1-P) and its derivatives. From the viewpoint that enzymatic phosphorolysis of dextrin is promising as an industrial production of G-1-P, kinetics of the enzymatic reaction in high concentration of substrate was investigated. Phosphorylase with a specific activity of 22.5 U(.)mg(-1.)protein was purified from potato. Linear chain dextrin and branched dextrin were used as substrate. For inorganic phosphate (Pi), a mixture of KH2PO4 and K2HPO4 was used. The initial rate of phosphorolysis was a function of the non-reducing-end concentration of alpha-1,4-glucan, regardless of the chain length and branch ratio of alpha-1,4-glucan. The inhibitory effects of both dextrin and phosphate were observed. From these experimental results, a possible kinetic model of the enzymatic reaction under high concentration of substrate was proposed. This model could well explain the experimental data obtained up to 18 mM of non-reducing-end of dextrin and 1000 mM of Pi.