Intraperitoneal administration of a single dose (6 mg/kg body weight) of mercuric chloride (HgCl2) results in ultrastructural changes in brain cortex of the rats. 18 hours after administration of HgCl2 the accumulation of dense deposits of mercury in nerve and glial cell cytoplasm was observed. The quantity of microglia in neuropil was also increased in this experimental group. We postulate that these cerebral macrophages can play an important role in the process of intoxication, too. 5 days after administration of mercuric chloride many of these changes reversed.