It has been suggested that continued diffusion gradients from mother to fetus would exist and that both meperidine and normeperidine would accumulate in the fetus following multiple doses of meperidine to the mother during labor. However, no pharmacokinetic data are available. Therefore, the purpose of this study was to document the disposition of meperidine and normeperidine in the fetus and neonate following multiple doses of meperidine to the mother over long time periods. Twelve infants were studied. The results show surprisingly high concentrations of both meperidine and normeperidine in fetal blood at delivery. In addition, the amount of normeperidine increased with time in umbilical cord blood, the ratio of normeperidine to meperidine increased with time, and the umbilical artery-to-vein ratio of meperidine (but not normeperidine) was greater than one following long drug-to-delivery intervals. The data also suggest that with long drug-to-delivery intervals the levels of normeperidine may become clinically important and that the elimination of both compounds by the neonate is prolonged. The study suggests that multiple doses to the mother over long time periods result in maximum accumulation of both meperidine and normeperidine in fetal tissues.