Measuring PaO2 in newborn infants receiving oxygen therapy is routine. The most reliable method requires intermittent sampling from an umbilical artery catheter or from a peripheral artery for analysis with a micro blood gas analyzer. Methods to continuously measure PaO2 using indwelling catheters have been developed but many of them receive limited use because of inaccuracy, tissue injury, clotting and interference with the function for which catheters are intended. Oxygen measuring sensors which can be applied to the skin have been developed by others. This technique measures PaO2 indirectly and can thereby serve only as a supplement for an accurate direct measuring method. We have devised a polarographic catheter electrode which is simply designed and made of materials with minimal thrombogenic activity. It does not interfere with blood pressure measurement or blood sampling through an end hole lumen. Our sensor is easily calibrated, accurate, and requires recalibration at 6-12 hour intervals. Micro clot analysis, aortic angiography, scanning electron microscopy of sensors and vessels, and haptoglobin determinations in baboons have shown minimal thrombus formation and red cell injury. The sensor responds rapidly and accurately to changes in PaO2 in newborn infants and adults. It reveals clinically inapparent changes of PaO2 caused by manipulation of the patient or respiratory equipment. A technique for continuously measuring PaO2 has been developed which can complement and may substitute for laboratory techniques.