The Sprague-Dawley rats with postnatal 3 d were collected.The hypetphenylalaninemia was induced by intraperitoneal injection with phenylalanine.Fluorospectrophotometry was applied to quantive determination of norepinephrine(NE),dopamine(DA)and serotonin(5-HT)in the cerebral cortex and its synaptosome of the rats. Behavior in the rats was observed by Y-maze learning.The results showed that compare with the control,the contents of NE,DA and 5-HT were decreased over 38.6%～67.4% in the cortex of the hyperphenylalaninemic rats and decreased over 51.9%～70.2% in the synaptosome of the cortex. Y-maze learning ability in the rats was decreased apparently. The results suggest that the mental retardation of PKU is in close correlative with decrease of some monoamine transmitter contents in the cortex and its synaptosome.