An electrochemical oxygen sensor using a smooth glassy carbon electrode for continuous monitoring of oxygen has been developed, which can find a wide range of application. Two different potential levels of preset durations are imposed on the sensing electrode in a cyclic manner. The current flow due to O2 reduction at a negative potential is integrated after a time lag. The charge is proportional to the oxygen concentration. The electrode is then held at a positive potential for a longer period before repeating the cycle. Thus, oxygen is consumed only during the short pulse duration. This sensor principle is unique, since the sensing electrode operates without a covering membrane. This is possible because of the low electrocatalytic activity of glassy carbon. The response is immediate and reproducible even at a low consumption of oxygen. The long-term response is stable over 60 h and sensitive to a change of about 0.5 hPa partial pressure of oxygen in saline solution. Measurements with the sensor in bovine serum demonstrated the negligible influence of other substances on the response to oxygen. The sensor has been tested in animal experiments, and shows a good response in blood as well as in tissue.