Uptake and transport in the olfactory neurons may be an important means by which some heavy metals gain access to the brain. In the present study we explored whether inorganic mercury (203Hg2+) may be taken up in the CNS via the olfactory pathway. Autoradiography and gamma spectrometry showed that intranasal instillation of 203Hg2+ in the right nostrils of rats resulted in much higher levels of the metal in the right olfactory bulbs than in the left ones. At the side of the application of the 203Hg2+ there was also a labeling of the olfactory nerve bundles projecting to the olfactory bulbs as well as in the olfactory nerve-fibres constituting the olfactory nerve layer of the bulbs, which was not seen on the opposite side. The results also showed that the 203Hg2+ accumulated in the glomerular layer of the bulbs. These data indicate that our results can be ascribed to a movement of the mercury along the olfactory axons to their terminal parts in the glomeruli and not to circulatory uptake from the mucosal vasculature. At late survival intervals a low labeling was also discernable in the external plexiform layer, indicating that a low level of 203Hg2+ leaves the terminal arborizations of the axons in the glomeruli. An uptake of 203Hg2+ in the glomerular layer of the olfactory bulbs was also seen in rats given the metal intraperitoneally. This uptake was similar in the right and left bulbs and always much lower than in the right bulbs of the rats given 203Hg2+ in the right nostrils. The intraperitoneal injections in addition resulted in an uptake of the 203Hg2+ in the olfactory epithelium. We propose that in these rats the mercury is taken up from the blood into the olfactory neurons and then moves along the axons to their terminations in the olfactory bulbs. In humans a continuous exposure of the nasal cavity to mercury vapor (Hg0), released from amalgam fillings and oxidized to Hg2+ in the olfactory mucosa, as well as a potential uptake of Hg2+ in the olfactory neurons from the blood, may lead to considerable concentrations of the metal in the olfactory bulbs.