Two successive intravenous doses of carbon suspension were given at 24 hourly intervals into six days old rats. These animals were killed at intervals ranging from 1 to 9 days after the second injection. The corpus callosum and neighbouring structures were examined for cells containing ingested colloidal carbon particles in their cytoplasm. Twenty four hours after the second injection, a variable number of carbon-labelled monocytes were adherent to the luminal wall of blood vessels in the corpus callosum. Numerous carbon-labelled cells appeared to have left the lumen and entered the brain tissue surrounding the vessels. These perivascular carbon-labelled monocytes in the neuropil displayed a large pale nucleus with fine chromatin granules. The phagocytic amoeboid microglia in the corpus callosum were unlabelled at first, although a few cells of a similar nature in the cavum septi pellucidi did show carbon particles in their cytoplasm. Four or five days after the second carbon injection perivascular carbon-labelled monocytes were rare, but carbon particles were now present in the amoeboid microglia. At 8 days amoeboid microglia were virtually absent from the corpus callosum but carbon particles now appeared in cells which closely resembled microglia (flattened nucleus, coarse chromatin, scanty cytoplasm at one pole). The sequential appearance of carbon particles in monocytes, amoeboid microglia, and microglia, suggests that monocytes transform into microglia by way of an amoeboid microglial stage.