Arterial partial pressure of oxygen (PaO2) can be measured continuously either directly in the blood (in an artery or in an arterio-venous shunt), or indirectly, using the transcutaneous method or by measuring respiratory oxygen. Construction of miniaturized PO2 electrodes according to the polarographic principle (1.2 and 2 mm in diameter) with ring-shaped cathodes, allowing fast assession of arterial PO2 with little sensitivity to blood flow, and its clinical application are described. An exhausting investigation of the electrod properties, including in vivo parameters allowed continuous monitoring of PaO2 either directly in the radial artery or more often in an arterio-venous shunt under clinical routine circumstances in a large number of patients up to 24 hours. The recordings obtained with both methods are identical and show three types of oscillations.