In 28 children undergoing cardiopulmonary bypass with deep hypothermia for open heart surgery, an attempt was made to maintain pH at 7.4 not corrected for temperature by varying the CO2 concentration supplied to the oxygenator so that the PaCO2 was 5.33 kPa, not corrected for temperature. One to two percent CO2 gave satisfactory results. Five percent CO2 had previously been given. No adverse clinical side effects were noted, and the acid-base status remainpd stable for 24 hours in 16 patients. There are strong theoretical reasons for maintaining a pH of 7.4, uncorrected for temperature, during hypothermia and a clinical impression was gained of better myocardial function and improved systemic and cerebral perfusion.