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Total mercury (Hg) profiles of sediment cores from lakes serve as historical records of Hg pollution. They do not, however, provide information about temporal variations in the abundance of the toxic, preferentially bioaccumulated species methylmercury (CH3Hg+), whose production rate is largely a function of environmental variables that control the growth and activities of Hg-methylating and Hg-demethylating bacteria and the bioavailability of the inorganic Hg(II) from which CH3Hg+ is formed. Earlier studies showed that CH(3)Hg(+)in freshwater food webs had anomalously high delta Hg-201/delta(199) ratios and, similarly, that the delta Hg-201/delta(199) ratio of Hg in a core from a severely Hg-polluted lake paralleled temporal changes in CH3Hg+ production. The present study was undertaken to test the hypothesis that variations in the delta Hg-201/delta(199) ratios of sediment cores from lakes reflect historical variations in CH3Hg+ production in the lakes. Core profiles of delta Hg-201/delta(199) ratios and total Hg concentrations from three widely separated, environmentally distinct temperate lakes polluted with Hg from different industrial sources, and one practically pristine Arctic lake, were examined, and changes attributable to known historical events were noted. Hg pollution led to increases in the delta Hg-201/delta(199) ratio, suggesting enhanced production of CH3Hg+, but the lakes showed different patterns of variation over time, implying the influence of site-specific environmental and biotic-factors. In one lake the delta Hg-201/delta(199) ratio increased at the onset of each of three pollution episodes but peaked and then declined, suggesting a surge in methylation that was reversed by demethylation. In another lake, mercury pollution initiated a long-term upward trend in the isotope ratio, with secondary maxima and minima superimposed on it, implying a progressive increase in CH3Hg+, with successive shorter-term changes caused by fluctuations in methylation and demethylation rates. In the third lake, moderately elevated isotope ratios and one large maximum were associated with Hg contamination, but the maximum is attributable to accelerated eutrophication that intensified CH3Hg+ production. In the Arctic lake, the isotope ratio varied with climate change, decreasing during warming trends and increasing during cooling phases. These results constitute evidence that delta Hg-201/delta(199) ratios of sediment cores from lakes record historical variations in the net rate of CH(3)Hg(+)production.