Bilirubin neurotoxicity can be mediated by numerous mechanisms due to its increased permeability in neuronal membranes. The present study tests the hypothesis that a prolonged bilirubin infusion modifies theN-methyl-D-aspartate (NMDA) receptor/ion channel complex in the cerebral cortex of newborn piglets. Studies were performed in seven control and six bilirubin-exposed piglets, 2-4 d of age. Piglets in the bilirubin group received a 35 mg/kg bolus of bilirubin followed by a 4-h infusion (25 mg/kg/h) of a buffer solution containing 0.1 N NaOH, 5% human albumin, and 0.055 Na2HPO4with 3 mg/mL bilirubin. The final mean bilirubin concentration in the bilirubin group was 495.9 ± 85.5 μmol/L (29.0± 5.0 mg/dL). The control group received a bilirubin-free buffer solution. Sulfisoxazole was administered to animals in both groups. P2membrane fractions were prepared from the cerebral cortex. [3H]MK-801 binding assays were performed to study NMDA receptor modification. TheBmaxin the control and bilirubin groups were 1.20 ± 0.10 (mean ± SD) and 1.32 ± 0.14 pmol/mg protein, respectively. The value forKdin the control brains was 6.97± 0.80 nM compared with 4.80 ± 0.28 nM in the bilirubin-exposed brains (p< 0.001). [3H]Glutamate binding studies did not show a significant difference in theBmaxandKdfor the NMDA-specific glutamate site in the two groups. The results show thatin vivoexposure to bilirubin increases the affinity of the receptor (decreasedKd) for [3H]MK-801, indicating that bilirubin modifies the function of the NMDA receptor/ion channel complex in the brain of the newborn piglet. We speculate that the affinity of bilirubin for neuronal membranes leads to bilirubin-mediated neurotoxicity, resulting in either short- or long-term disruption of neuronal function.Abbreviations: NMDA,N-methyl-D-aspartate;Bmax, total number of receptors;Pao2, partial pressure of arterial O2;Paco2, partial pressure of arterial CO2