The changes in arterial carbon dioxide tension (PaCO2) during cardiopulmonary bypass reflect changes of temperature and gas flow through an oxygenator. The changes in PaCO2 can be reduced through the frequent analysis of arterial blood gases and subsequent adjustment of total gas flow and CO2 concentration in the gas flow or both. Utilizing a capnometer (CAPNOMAC AGM-103. Datex), we compared end-tidal carbon dioxide tension (PetCO2) from the capnometer with temperature corrected PaCO2 during cardiopulmonary bypass. One end of the sampling port of the capnometer was incorporated into the prime port of the arterial reservoir in a bubbling type oxygenator (William-Harvey). When arterial reservoir temperatures of the oxygenator were 30 degrees C and 35 degrees C, PetCO2 from the capnometer was recorded and two arterial blood gas samplings were done at the same temperatures. The results were as follows: 1) The difference of PetCO2 and temperature corrected PaCO2 was below 3 mmHg in all cases. 2) The relationship between PetCO2 and temperature corrected PaCO2 was significantly linear. The results show that continuous monitoring of PetCO2, using a capnometer is useful to control the changes in PaCO2 during cardiopulmonary bypass.