Some investigators have abandoned the use of 203Hg emulsion autoradiography in favor of chemical methods of microscopic localization of mercury. However, recent studies indicate that the latter methods identify only inorganic mercury, or some product of inorganic mercury, making them of little or no value for studies of methylmercury toxicity. Doubts about the use of 203Hg for microscopic localization arose because of the high maximum energy of its emissions and the concern that its latent images might be confounded with silver grains produced by chemical reactions between tissue Hg and the silver supplied by photographic emulsions. Examination of the spectrum of emissions from 203Hg demonstrates that its maximum energy emissions are rare. The mean energy of 203Hg emissions is in the 50-ke V range and the modal emissions are close to 0 ke V, indicating sufficient low energy emissions for autoradiography. In preliminary experiments, methylmercury content of mouse brain was shown to be stable through the steps of tissue processing for plastic sections. A direct comparison of autoradiographic grain counts from tissue treated with "cold" or "hot" methylmercury demonstrated that no grains above background were produced in the absence of nuclear emissions--only "hot" samples affected emulsion. In the kidneys of mice killed 24 hr after dosing, grains were most numerous over cortical tubules and significantly less numerous over glomeruli. In the cerebellum, the molecular layer was significantly more heavily labeled than the granular layer. The number of grains was greatly increased in every region by increasing the specific activity of the methylmercury dosing solution while holding the dose of methylmercury constant. Like the differential effect of "hot" vs "cold" tissue, the differential effect of low vs high specific activity confirms that the grain counts reflect nuclear emissions from the sample tissues, rather than a chemical effect dependent only on mercury content. Grain counts provided a measure of methylmercury content that matched the content measured by atomic absorption (AA). For example, the ratio of kidney/brain content was 32 by AA and 31 by grain counts in one experiment. Thus, 203Hg emulsion autoradiography appears to be a useful approach to localization of methylmercury in tissue sections processed for light microscopy.