Phosphorylase activity of the brain of rats from newborn to adults was investigated histochemically by Takeuchi and Kuriaki's method, using the iodine reaction. Additional studies were made of the brain of adult guinea pigs, mice and rabbits. The phosphorylase of newborn rats was generally slight in activity, except for moderate or strong action in some structures: arachnoid membrane, choroid plexuses, ventricular walls, area postrema, pineal and subcommissural bodies, raphe of the medulla oblongata, some brain stem nuclei such as the nucleus of the mesencephalic tract and motor nucleus of the fifth nerve, nuclei of the facial and hypoglossal nerves. Slight reaction appeared in the ventral thalamic and habenular nuclei on the 3rd day, in the cerebral and cerebellar cortices and superior colliculus on the 5th day after birth. The enzyme activity showed a general increase at 14 days and reached the maximal intensity at about 21 days of age, while the arachnoid membrane, choroid plexuses amid raphe exhibited evident decrease of activity. The white matter showed an initial activity at different ages from 5 to 21 days, to attain maximal activity by 1 month. In the adult a moderate purple staining was consistently demonstrated in the area postrema, subcommissural body and ventricular walls. The neopallium as well as the superior colliculus and cerebellar cortex showed strong or moderate brownish purple color. Nuclei of the brain stem gave characteristic staining; a purple color was encountered in the nucleus of the mesencephalic tract and motor nucleus of the fifth nerve, nuclei of the facial and hypoglossal nerves, brown or purplish brown color occurring in the caudate nucleus, putamen, interpeduncular and inferior olivary nuclei.