The pharmacokinetics of fentanyl (F) were studied in 10 children, age 5 months-4.5 yr (mean 19 months) undergoing cardiac surgery with cardiopulmonary bypass ( CPBP ). They suffered from transposition of the great arteries (6), tetralogy of Fallot (2), and atrio-ventricular (A-V) canal (2). Induction of anesthesia included a bolus of 50 micrograms X kg-1 X min-1 F followed by a continuous F infusion of either 0.15 micrograms X kg-1 X min-1 (4 patients) or 0.3 micrograms X kg-1 X min-1 (6 patients). The F infusion was discontinued when cardiopulmonary bypass was started, 81-141 min (mean 112 min) along with deep hypothermia. Blood was collected throughout surgery from an indwelling radial arterial catheter and plasma concentration of F was assayed by GLC. F plasma concentrations after 30 min were 2-3-fold higher than reported with the same regimen in adults. The calculated values for t1/2 alpha (12 +/- 9 min) (mean +/- SD), t1/2 beta (141 +/- 98 min) and total body clearance (12.8 +/- 7.3 ml X min-1 X kg-1) were similar to adult values. The significantly lower steady-state volume of distribution observed in children with intracardiac shunts (1385 +/- 875 ml X kg-1) compared to reported values for adults (3200-6000 ml X kg-1) explains the higher F plasma concentrations achieved in these children. Cardiopulmonary bypass produced a mean 70% (range, 56-89%) decrease in plasma F, significantly higher than would be expected from hemodilution alone. Studies of F disposition in the CPBP demonstrated that F is bound to the pump.(ABSTRACT TRUNCATED AT 250 WORDS)