Monitoring blood glucose level is an important procedure for any diabetic subject. The blood, however, has oxygen tension less than 5%. Since the oxygen in the sensor membrane is consumed by the enzyme reaction, the oxygen concentration is needed high enough for a better linearity between output current and the glucose concentration. In this monitor, the oxygen is generated by electrolysis of the solution before monitoring, and the glucose concentration is monitored by a peak current obtained by a potential sweep method. A theoretical analysis was made considering diffusions of glucose. oxygen and hydrogen peroxide, and also the rate of the two-substrate reaction catalyzed by glucose oxidase (GOD) within the enzyme membrane. Concentrations within membranes were analysed by a model of three layers of the solution, the overcoat gel and the enzyme membrane. The output current increased with time after sweeping the potential anodically and had its maximum in 10-20 s. The peak current was linearly proportional to the glucose concentration up to 500 mg dl(-1) by the supplement of oxygen generated by electrolysis of the solution. The peak current was well simulated by a mathematical model. When one minute is adopted to this monitor, the optimum conditions of M-G, enzyme membrane thickness and electrolysis time were determined to be 1.9-3.8, 4.7-9.4 mu m and 5-10 s, respectively.