Mercury contamination of fish is a serious problem, and consideration of policy options for a solution requires answering two questions. (1) What is the evidence that fish are being adversely affected by mercury? (2) How would mercury concentrations in fish respond to reduced anthropogenic emissions of mercury? I performed research integral to answering these questions. First, I studied mercury accumulation in northwest Atlantic sea lamprey, an agnathan fish. I discovered there are fundamental differences in mercury accumulation between sea lamprey and teleosts. All life stages of sea lamprey are exposed to high concentrations of mercury. More research should be directed towards understanding mercury contamination of diverse groups of fishes and its toxicological effects. Second, I studied the toxicological mechanisms of methylmercury in fish. Exposure to methylmercury impairs the reproduction of fish, and previous research has linked impaired reproductin in fish to the suppression of sex steroid hormones. I exposed fathead minnows to dietary methylmercury to test the hypothesis that methylmercury induces apoptosis in steroidogenic cells in fish, thereby interfering with the synthesis of sex steroid hormones. Methylmercury increased the number of apoptotic follicular cells in ovarian follicles, which in turn was related to suppressed 17B-estradiol concentrations in females. These results ultimately suggest apoptosis of steroidogenic cells as a mechanism for the impairment of reproduction in fish exposed to methylmercury. Third, I studied the resopnse of mercury contamination in fish to changing atmospheric pollution. Serendipitously, I discovered a decline during the past decade in mercury concentrations in fish at Isle Royale, USA. Changes in mercury source have not caused the decline, as deposition has reamined stable during the past decade. I examined sulfur concentrations in lake sediments to test the hypothesis that declines in sulfate deposition have resulted in less methylation of mercury and thus less mercury in fish. Results indicated mercury concentrations in fish are primarily a function of sulfate deposition. In sum, (1) fish are adversely affected by methylmercury and (2) mercury concentrations in fish will decline in response to reduced anthropogenic emissions of mercury and sulfur.