The aim of this study was to measure the effects of extracorporeal circulation (ECC) on lidocaine disposition and to determine if therapeutic concentrations were achieved during ECC. Anesthesia was obtained by administration of fentanyl (50 microg/kg plus 0.3 microg/kg/min) and pancuronium bromide (0.1 mg/kg) along with controlled ventilation (10 mL/kg, PaCO2, 30 to 34 mmHg). Lidocaine was administered to ten patients in bolus doses of 2 mg/kg at the time of endotracheal intubation and during ECC at aortic declamping. Samples were collected at 20 and 40 seconds, and at 1, 3, 5, 10, 15, 20, 25, and 30 minutes after drug administration. Blood sampling was arterial during anesthesia induction and from the oxygenator during ECC. Blood levels of lidocaine were determined by the immunofluorescence method. The kinetic studies of lidocaine showed increases in plasma volume and in the volume of the peripheral compartment, along with decreased volume in the compartment of well-perfused tissues during ECC. The fraction of the dose of lidocaine administered during ECC behaved accordingly, showing a decrease in the quantity of drug in the well-perfused tissues, and an increase in the other two compartments when compared to values before ECC. The doses administered gave plasma concentrations of 1.55 microg/mL, which are considered therapeutic. Thus, it is not recommended to increase the lidocaine dosage during ECC.