The effects on the salmon (Salmo salar L.) electroolfactogram (EOG) of the two mercurials, mercuric chloride (HgCl2) and methylmercuric chloride (CH3HgCl), were studied. The EOG responses were evoked by stimulating the olfactory epithelium with 340 microM L-alanine for 10 sec every second minute during a 1-hr period. Each EOG response consisted of an initial peak component followed by a sustained component with an amplitude about 40% below the peak value. Three experimental series, each comprising six fish, were carried out. In the first series, the rosette was irrigated solely with artificial "fresh water." In the two other series, a 5-min exposure to mercury (HgCl2 or CH3HgCl, at 10(-5) M) was included after 10 min and a 15-min exposure after 45 min. The mercuric ion (Hg2+) eliminated the peak response within 2 min and suppressed the sustained response to about 35%. During the subsequent irrigation with mercury-free fresh water, both EOG components regained about 50% of their initial amplitudes. In contrast, methylmercury induced a steady and parallel decline of both the peak and the sustained responses, which were not reversed by rinsing the epithelium with fresh water. The results of this study demonstrate the vulnerability of the olfactory receptor function in fish to mercury exposure. Also, they show the very different effects of inorganic and organic mercurials upon the EOG.