To determine whether paralysis reduces oxygen consumption (VO2) after cardiac surgery in infants, the authors measured VO2before and after paralysis in 17 sedated infants who were ventilated mechanically after cardiac surgery. Oxygen consumption was determined as being the difference between oxygen content of inspired and expired gases. The absence or presence of “movement” (breathing or repeated movement of the extremities) before paralysis was noted. For eight infants who did not “move” before paralysis, VO2was similar before (9.1 ± 1.2 ml · kg-1· min-1, mean ± SD) and after (9.0 ± 1.5 ml · kg-1· min-1) paralysis (P= 0.81). However, for nine infants who did “move” before paralysis, VO2decreased from 9.2 ± 1.4 ml · kg-1· min-1before paralysis to 8.0 ± 1.4 ml · kg-1· min-1after paralysis (P< 0.05). One infant in each group had an increase in VO2greater than 10% of the baseline value (i.e., 12% and 14%). In conclusion, if breathing or repeated movement is present before paralysis, paralysis decreases VO2by 13% in sedated infants after cardiac surgery. If repeated or regular movement is not present before paralysis, paralysis does not decrease VO2. These data suggest that in normoxic patients, muscle paralysis does not significantly alter VO2and therefore should not be used for this purpose.