The aim of this study was to evaluate the usefulness of continuous gas monitoring during haemodialysis using an oxygen- and carbon dioxide-sensitive electrode inserted in the 'arterial' line from the arteriovenous fistula. We investigated 15 patients with chronic renal failure, treated with regular bicarbonate haemodialysis. The PO2 and PCO2 were measured by the new method and compared to conventional blood gas analysis of the 'arterial' blood from the dialysis circulation. We found the in situ gas sensors were stable and there was no systematic difference between gas tensions measured by in situ and conventional blood gas measurements. The standard deviations of the difference between paired measurements for PO2 and PCO2 were 1.3 and 0.4 kPa, respectively. Therefore, the method can be used to detect major changes in blood gas tensions during haemodialysis.