In 46 patients undergoing cardiac surgery blood PO2 was continuously monitored with an in-line oxygen electrode during cardiopulmonary bypass and readings were compared with intermittent measurements of samples on a blood gas analyser. In 35 patients in whom arterial PO2 was monitored, there were rapid changes in PO2 and the mean difference between the in-line electrode and the blood gas analyser was 6.63 kPa (p less than 0.001). Venous PO2 was monitored in 11 patients and, in contrast to the arterial readings, the venous PO2 was relatively stable. The mean difference between the in-line electrode readings and the blood gas analyser was 0.21 kPa (p less than 0.05). The advantages of continuous monitoring of blood PO2 are described and the relative merits of mixed venous PO2 and mixed venous oxygen saturation measurements are discussed.