An electron microscope study of the corpus callosum in postnatal rats of various ages was carried out to elucidate the fate of the amoeboid microglial cells. The cells present in the corpus callosum of younger rats (3-5 days) were round and showed an eccentric nucleus with marginal chromatin. They displayed numerous lysosomal granules and vacuoles in the cytoplasm. In older animals, i. e., from 7 days onwards some of the cells became oval so that by 15-20 days of age most of the cells were elongated and branched. In the latter, the cells showed a flattened or angular nucleus with dense chromatin clumps. The cytoplasm showed fewer lysosomal granules and vacuoles which were absent in cells of 20 day old animals. Quantitative measurements showed that there was a gradual diminution in the amount of cytoplasm at the cell body of amoeboid microglial cells with age, so that by the age of 20 days the cells were reduced to less than one-third of their original size as seen in 3 day old rats. The reduction of cytoplasm at the cell body is probably because it is channelled to the cytoplasmic processes which are evident in older rats. Some cytoplasm may have been extruded and phagocytosed by companion cell types.