Abstract1) Our investigation was made on the distribution of the free amino acids in the five parts of brain. The anomalies of these distributions under the influence of premature birth have recovered with growth, but the anomalies have been found to be detectable in the diencephalon and the midbrain even at the third week of life.2) By using the ratio' of (Glu+GABA+Asp)/(Tau+PE) as the maturity index, it is found that the premature rat is retarded in development in all parts of brain except the cerebrum and is remarkably retarded in the diencephalon.3) The metabolism of sulfur‐containing amino acid in the brain of premature rats was found to be more active than that in control rats at the time of birth. The metabolism of the premature rat had caught up with that of the control rat at the age of thre