Protein phosphorylation in cerebral cell-free preparations from neonate rabbits was inhibited by bilirubin and promoted by aminophylline when these substances had been administered intravenously. In animals given both compounds, the bilirubin-induced inhibition of phosphorylation was partly reversed by aminophylline. Adenosine 3',5'-monophosphate added in vitro during the assays also increased protein phosphorylation. These data introduce new concepts in the pathogenesis of kernicterus.