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SummaryWe investigated blood flow and regional oxygenation (rSO 2 ) during cardiopulmonary bypass (CPB). Twenty infants (mean (SD) age 5 (3) months, weight 5.4 (1.6) kg) were prospectively studied. Total CPB and superior vena cava (SVC) flow were measured using Transonic Bypass Flowmeters, inferior vena cava (IVC) flow derived arithmetically and rSO 2 measured using Near Infra-Red Spectroscopy. Mean SVC flow was 51.3 (14.8) ml.kg .min )1 . Mean cerebral rSO 2 was 71 (11)% and somatic rSO 2 55 (13)%. Cerebral and somatic rSO 2 showed no correlation with SVC and IVC flow. Cerebral rSO 2 showed a positive correlation with P a CO 2 , mean arterial pressure (MAP) and haematocrit (p < 0.0001). Somatic rSO 2 showed a positive correlation with MAP and haematocrit (p = 0.01, p = 0.02).In conclusion, the distribution of blood flow during CPB varies. The most important factor affecting this is P a CO 2 . Cerebral and somatic oxygenation are unaffected by flow but significantly influenced by MAP, haematocrit and P a CO 2 .