Objective: To evaluate the use of ketamine in comparison with isoflurane in the maintenance of anesthesia in children with tetralogy of Fallot. Design: Prospective, randomized study. Setting: University hospital. Participants: Fifty children scheduled for correction of tetralogy of Fallot. Interventions.. After intubation, patients were assigned randomly to receive 2 different anesthesia maintenance regimens: group I, isoflurane, 0 to 1% plus fentanyl. 0.1 mug/kg/ min; group II, ketamine, 0 to 5 mg/kg/h, plus fentanyl, 0.1 mu g/kg/min. Isoflurane concentration and ketamine infusion rate were adjusted to maintain arterial pressure within 25% of baseline. Hemodynamic and respiratory parameters were recorded at the end of 4 intervals: TO, before induction of anesthesia; T1, induction to 10 minutes postintubation; T2, 10 minutes postintubation to poststernotomy: and T3, poststernotomy to completion of catheterizations. Measurements and Main Results: In comparing group I with group II, significant differences were observed in mean arterial pressure (p < 0.0001), heart rate (p < 0.01), arterial oxygen saturation (p < 0.0001), arterial oxygen tension (p < 0.001). arterial carbon dioxide tension (p < 0.001). arterial pH (p < 0.0001), base excess (p < 0.05), and arterial to end-tidal carbon dioxide tension difference (p < 0.01) at T3. Conclusion: The use of ketamine anesthesia is recommended as an alternative maintenance regimen in children undergoing definitive correction of tetralogy of Fallot. Copyright (C) 2000 by W.B. Saunders Company.