Preterm Gunn rat model of neonatal hyperbilirubinemia (NHB) was used to investigate effects of unconjugated bilirubin (UCB) on the developing hippocampus and cerebellum. In this model, sulfadimethoxine was injected (i.p.) to Gunn rat pups on postnatal day 5 to exaggerate NHB effects at a preterm equivalent age. 1H MRS and MRI results clearly demonstrate that high levels of UCB in early postnatal age critically affects the brain development in a region-specific manner. In this model, cerebellum appears to be much more vulnerable to high levels of UCB than hippocampus resulting in abnormal postnatal development.