Previous work has shown that both meperidine and normeperidine are transferred across the placenta to the fetus. Little is known in primates, however, about the tissue deposition of these compounds. Four pregnant, dated rhesus monkeys within one week of term were anesthetized for cesarean delivery. An equal mixture of meperidine and normeperidine was administered as an intravenous bolus 10 minutes before delivery (1.25 mg/kg). The infants were then sacrificed at 20 minutes after birth and the concentration of the compounds in various organ systems were analyzed by gas-liquid chromatography and mass spectroscopy (GLC-MS). The infant serum 20 minutes after delivery revealed a meperidine concentration of 2.23 micrograms/ml and a normeperidine level of 0.67 micrograms/ml (3:1). In contrast, the tissues analyzed showed a much higher concentration of the metabolite in the liver (1:7), gallbladder (1:3), and brain (1:2). Other tissues, such as muscle and kidney, demonstrated equal levels of the two compounds. The authors conclude that normeperidine is quickly transferred to fetal tissues and to a greater degree than the parent compound in certain organs. The increased distribution, particularly in the brain, could account for the toxic actions in the cerebrum of the derivatives of meperidine.