Experiments in Gunn rats have provided important insights into physiological, biochemical, and molecular mechanisms of bilirubin conjugation and disposition. The Gunn rat is a natural model for bilirubin encephalopathy, and much of our knowledge of bilirubin toxicity and its treatment has come from studies performed in Gunn rats. The genetic lesion in Gunn rats closely parallel those in Crigler-Najjar syndrome, Type I. Presently, the Gunn rat model is being used to develop methods for gene therapy for inherited bilirubin-UGT deficiency. Thus, Gunn rats continue to be a valuable model for the investigation of inherited UGT deficiency and severe nonhemolytic unconjugated hyperbilirubinemia.