Slide-mounted 12-microns cryostat sections of rat brain were incubated with [3H]bilirubin, washed, dried, and apposed to emulsion-coated coverslips for autoradiography. The binding appeared to be nonsaturable. Within the gray matter, binding was concentrated over neuronal cell bodies (particularly evident over hippocampal pyramidal and granular cells, and over cerebellar Purkinje cells), suggesting that the preferential neuronal toxicity of bilirubin may be related to neuronal binding. Regional differences in bilirubin binding were not demonstrated.