The blood concentrations of lidocaine and its main active metabolites, methylethylglycinexylidide (MEGX) and glycinexylidide (GX), were measured in 24 newborn infants during anticonvulsive treatment with an iv infusion of lidocaine. After a bolus dose of 1.5-2.2 mg/kg and continuous infusion of lidocaine (4.7-6.3 mg/kg/h) there was accumulation of the drug and MEGX within 24 h. After termination of the iv infusion, both lidocaine and the metabolites were eliminated within 24-48 h. The anticonvulsive effectiveness--estimated by clinical observation and continuous amplitude integrated EEG monitoring (cerebral function monitor)--was immediate in 15 infants (nine term and six preterm). There was no correlation between blood concentrations of lidocaine and metabolites, and anticonvulsive effect (i.e. good, intermediate or no response). No differences in blood concentrations were found between full-term and preterm babies, or between infants with or without birth asphyxia. In combination with a fast withdrawal of the drug, few adverse reactions were seen with the dosages used, even though blood concentrations were high. Routine measurements of lidocaine concentrations during anticonvulsive treatment in neonates seem to be of little clinical value. For evaluation of the anticonvulsive effect and for early detection of seizure activity during lidocaine withdrawal, continuous EEG monitoring is preferable.