Selenium has been suggested to enhance the histochemical staining of mercury when sections of tissue are subjected to the silver-enhancement method. In the present study, histochemical staining patterns of mercury in tissue sections of rat livers were compared with the actual content of organic and inorganic Hg in the livers, in both the presence and the absence of Se. Rats were injected intravenously with 5 micrograms of Hg g-1 body weight as methyl [203Hg] mercury chloride (MeHg) or as [203Hg]mercuric chloride (Hg2+). After 2 h, half the rats received an additional intraperitoneal injection of 2 micrograms of Se g-1 body weight as sodium [75Se]selenite. All the rats were killed 1 h later. Homogenized liver samples were prepared for mercury analysis by two different methods: alkaline digestion and ultrasonic disintegration. Quantitative chemical analysis based on benzene extraction of the radioactively labelled Hg compounds showed that the chemical form of mercury, either organic or inorganic, was preserved from its administration to its deposition in the liver. Light and electron microscopy demonstrated that no silver enhancement of Hg occurred when MeHg alone was present in the sections of tissue, whereas MeHg accompanied by Se induced a moderate deposition of silver grains. In contrast, sections containing Hg2+ alone yielded some staining, and the addition of Se increased the staining dramatically. The results of the present study show that acute selenite pretreatment is a prerequisite for the histochemical demonstration of methyl mercury, and greatly increases the staining of inorganic mercury when applying the silver-enhancement method.