Fentanyl-oxygen (fentanyl-O2) anaesthesia was compared to fentanyl-nitrous oxide/oxygen (fentanyl-N2O/O2) anaesthesia in 14 children undergoing cardiac surgery. Children were randomly assigned to one of the two techniques studied, with seven patients in each group. The mean age (mean +/- SE) was 3.9 +/- 0.75 years (0.5-8.25 years) and mean weight 14.7 +/- 2 kg (3.5-29.5 kg). Patients were premedicated with IM atropine 0.02 mg . kg-1 and morphine 0.2 mg . kg-1 1 hour preoperatively. They received a fentanyl bolus of 30 micrograms . kg-1 with a concomitant continuous infusion of 0.3 micrograms . kg-1 . min-1. Pancuronium 0.1 mg . kg-1 was administered immediately following the fentanyl bolus. Fifty per cent nitrous oxide was given with oxygen in one group and 100 per cent oxygen was administered to the other group. Fentanyl plasma concentrations were similar in the two groups at the various stages of surgery. There were no significant differences between the two treatment groups in systolic and diastolic blood pressure or in heart rate in response to induction, intubation, and incision. There was a significantly greater increase in systolic blood pressure after sternotomy in the fentanyl-O2 group. In addition, in six of seven patients receiving fentanyl-O2 there were events of sudden increase in blood pressure during various stages of surgery before the bypass, necessitating an additional fentanyl bolus or the addition of droperidol in four cases. Similar phenomena were not documented in the fentanyl-N2O/O2 group. Our studies suggest that fentanyl-O2 anaesthesia in the schedule described, in children undergoing elective cardiac surgery for Tetralogy of Fallot, A-V canal, and transposition of the great arteries, is not sufficient to prevent elevation in systolic blood pressure despite fentanyl plasma concentrations in excess of 20 ng X ml-1. The addition of nitrous oxide prevents this phenomenon.