The use of a combined electrochemical and fibreoptic continuous intra-arterial blood gas sensor is described. The purpose of the study was to evaluate the performance of the sensor in 10 patients in the intensive therapy unit following insertion through a femoral arterial cannula. To our knowledge this is the first published study on the valuation of an intravascular blood gas sensor through a femoral arterial cannula. A total of 71 sets of data comparing the sensor with the blood gas analyser were obtained. The bias and precision for pH, PCO2 and PO2 were 0.006 and 0.07 pH units, 0.2 and 1.65 kPa (4.6% and 29%) and 0.8 and 2.7 kPa (5.1% and 14.3%) respectively. There was a degree of imprecision of the PCO2 sensor, the reasons for which are discussed. In summary, the intra-arterial sensor functioned well when inserted into the femoral artery in post-cardiopulmonary bypass patients. There were no complications attributable to sensor placement.