AbstractThe concentration of glutamic acid decarboxylase (μmoles of glutamic acid decarboxylated/hr/gm) was found higher in the anterior midbrain than in several other regions of the adult rat brain. It was the highest in the anterior colliculus.A developmental study of the forebrain, cerebellum, anterior midbrain, and posterior midbrain showed that the four regions behaved as distinct organs as regards the deposition of structural constituents and the activity of glutamic acid decarboxylase. There was evidence of postnatal cell acquisition by the four regions. The rate of accretion of DNA, RNA, phospholipid, and protein was the highest in the cerebellum, but that of glutamic acid decarboxylase was highest in the forebrain during postnatal development. The results indicated that the average size of the cerebellar cell in the adult rat was smaller than that of the cell of the forebrain, anterior midbrain, and posterior midbrian. The phospholipid content compared to that of protein was higher in the posterior midbrain than in the other three brain regions.The presence of fully differentiated GABAergic neurons in the cerebellum appear by the end of the first postnatal week