The distribution and cellular localization of mercury in the in situ brain and upper cervical spinal cord of adult Wistar rats were studied at various time intervals after oral administration of methylmercuric chloride (CH3HgCl; 20 mg x liter-1). Coronal sections of the brain and transverse sections of the cervical spinal cord were prepared for visualization of the mercury by the autometallographic silver-enhancement method. Following mercury administration there was a latent period before the metal appeared in the tissue. Mercury staining was first detected after 10 days in cell bodies of five specific areas of the brain stem: the mesencephalic nucleus of the trigeminal nerve, the red nuclei, the ventral cochlear nucleus, the superior vestibular nucleus, and the nucleus reticularis pontis caudalis. After 28 days of treatment, a fairly even distribution of mercury was seen in the brain and spinal cord. Longer periods of treatment caused no further increase in the density of mercury within the stained cell bodies. In cerebral cortex, staining commenced in piriform and entorhinal cortices. This was followed by staining in neurons of lamina III in the isocortex and ultimately all layers were stained after 28 days of treatment. After 20 days of treatment, mercury deposits in the cerebellar cortex were restricted to Purkinje cells, Golgi epithelial cells, and Golgi cells, while in the spinal cord the majority of mercury was located in the anterior horn motoneurons. Scattered ependymal cells and epithelial cells of the choroid plexus also exhibited mercury staining. The principal target cells were neurons followed by the glial and ependymal cells. Ultrastructurally, the bulk of detectable mercury was localized in lysosomes.