In order to study gas exchange and metabolic rate in anaesthetized children scheduled for corrective cardiac surgery and to find out if chronic hypoxaemia influenced gas exchange and energy expenditure, oxygen consumption (V̇O2) and carbon dioxide elimination (Vdot;co2) were measured and energy expenditure (E) was calculated. Infants and children whose haematocrit (Hct) was less than 44% and arterial oxygen saturation (Sao2) on room‐air was greater than 93% were classified as acyanotic (group AC,n= 11, weight range 3.7 to 20 kg), and those whose Hct was higher than 44% andSao2less than 93% as cyanotic (group C,n= 14, weight range 3.4 to 24.3 kg). The majority of children in both groups weighed less than the 50th percentile for normal children. There was no difference inV̇o2,V̇co2andEbetween the groups. These variables were related to weight according to the following expressions:V̇o2(ml min−1) = 6.1 × kg + 21.6,r= 0.95;V̇co2(ml min−1) = 5.7 × kg + 2.9,r= 0.96, andE(kcal h−1) 1.8 × kg + 5.3;E(J h−1) = 7.6 × kg + 22.3,r= 0.96. Fluid volumes (FV) could be calculated according to the expression:FV(ml h−1) = 3.0 × kg + 8.7;r= 0.96. Oxygen consumption was 15 to 20% higher in anaesthetized infants and children with congenital heart malformations than in anaesthetized infants and children with normal cardiopulmonary function. Accordingly, energy expenditure and fluid requirements were also higher. This difference was most probably due to an undernutrition in children with congenital heart malformations which resulted in a compens