The purpose of this study was to determine the pharmacokinetics of lidocaine in children with congenital heart disease (CHD). Fifteen children with left to right intracardiac shunting of blood (acyanotic group) and 15 children with right to left intracardiac shunting of blood (cyanotic group) were studied and compared with 15 children without CHD (control group). Lidocaine (1.5 mg.kg-1) was injected into a peripheral vein over 30 sec and serial samples of arterial blood were obtained up to 120 min after completion of the infusion. Total and free lidocaine were analyzed by enzyme immunoassay. The serum concentration of alpha 1-acid glycoprotein (alpha 1-AGP) at induction of anaesthesia was measured in the three groups by radial immunodiffusion. The percent free lidocaine (100 x [free lidocaine]/[total lidocaine]) was greater at 30 sec post-infusion in all three groups (35-37%) than it was at any other time but was not significantly different among the three groups (P less than 0.05). There was no significant difference in either the percent free or the total lidocaine concentration at any sample time or in any of the pharmacokinetic variables among the three groups. The serum concentration of alpha 1-AGP did not differ significantly among the three groups of patients. We conclude that the presence of intracardiac shunts does not alter the pharmacokinetic behaviour of intravenous lidocaine (1.5 mg.kg-1) in children. The percent free lidocaine is greatest immediately post-injection and this may mitigate against rapid bolus administration of intravenous lidocaine in children.