['Phenylalanine has been considered the main responsible agent for the brain damage that occurs in phenylketonuria.', 'In this work we studied the effect of this amino acid on the in vitro phosphorylation of cytoskeletal proteins of the cerebral cortex of rats. We observed that 2 mM phenylalanine, a concentration usually found in the plasma of phenylketonuric patients, decreased the in vitro 32P incorporation into these proteins. In addition, we investigated the effect of alanine on the inhibition of 32P incorporation into cytoskeletal proteins caused by phenylalanine. We observed that 0.5 mM alanine did not alter 32P incorporation but prevented the inhibition provoked by phenylalanine.'] Is phenylalanine inhibition of the phosphorylation of cytoskeletal proteins from cerebral cortex of young rats prevented by alanine?