Arterial PO2, PCO2 and pH were measured continuously with electrodes placed in a flow cuvette. Comparison between continuous readings and results of single sample analyses showed practically no discrepancy, and the drifts of the electrodes during time of measurements were negligible. During hypothermic perfusion with a Rygg-Kyvsgård bubble oxygenator, autotransplantation of canine hearts was performed. PO2, PCO2 and pH were measured continuously to check the performance of the heart-lung machine, and to evaluate the therapeutic and diagnostic significance of these blood gas values during cardiac surgery. At the start of perfusion, a steep fall in all three parameters was observed. The average fall in PO2 was 313 mmHg; PCO2 fell by 15 mmHg and pH BY 0.15. PCO2 rapidly returned to normal values, while pH and PO2 increased slowly during the perfusion period. PO2 reached its highest value at the lowest temperature and fell during rewarming. PCO was regulated by the carbon dioxide concentration in the heart-lung machine. pH did not return to normal levels within the time of perfusion. In the transistional period from perfusion, PCO2 increased and pH fell. Alterations in the distribution and direction of blood flow and a low systemic blood pressure are possible explanations of the initial fall in PO2 and the post-perfusion changes in PCO2 and pH. The variations in pH and PCO2 at the start of perfusion were caused by an acid priming fluid with low CO2 content. The post-perfusion changes indicated an unstable circulation, but imminent myocardial failure could not alone be diagnosed by continuous measurement.