We investigated the metabolism of lidocaine to its active metabolites--monoethylglycinexylidide (MEGX) and glycinexylidide (GX)--in the mother, fetus, and neonate. The study population included normal patients and their infants delivered either vaginally or by cesarean section. A group of infants of mothers in whom pudendal anesthesia was induced was also included. Using gas chromatography and mass spectrometry techniques, the concentrations of lidocaine, MEGX, and GX were determined in maternal plasma during labor, in umbilical cord venous and arterial plasma at delivery, and in maternal and neonatal urine for 3 days post partum. The results indicate the following: In maternal plasma, MEGX rises throughout labor and GX is usually detectable within an hour of medication; in cord blood plasma the levels of lidocaine, MEGX, and GX suggest fetal metabolism of lidocaine; and in neonatal urine, the relative levels of parent compound and metabolites confirm lidocaine metabolism by the neonate.