Mercury contamination of the environment from historical and ongoing mining practices that rely on mercury amalgamation for gold extraction is widespread. Contamination was particularly severe in the immediate vicinity of gold extraction and refining operations; however, mercury--especially in the form of water-soluble methylmercury--may be transported to pristine areas by rainwater, water currents, deforestation, volatilization, and other vectors. Examples of gold mining-associated mercury pollution are shown for Canada, the United States, Africa, China, the Philippines, Siberia, and South America. In parts of Brazil, for example, mercury concentrations in all abiotic materials, plants, and animals--including endangered species of mammals and reptiles--collected near ongoing mercury-amalgamation gold mining sites were far in excess of allowable mercury levels promulgated by regulatory agencies for the protection of human health and natural resources. Although health authorities in Brazil are unable to detect conclusive evidence of human mercury intoxication, the potential exists in the absence of mitigation for epidemic mercury poisoning of the mining population and environs. In the United States, environmental mercury contamination is mostly from historical gold mining practices, and portions of Nevada remain sufficiently mercury-contaminated to pose a hazard to reproduction of carnivorous fishes and fish-eating birds. Concentrations of total mercury lethal to sensitive representative natural resources range from 0.1 to 2.0 ug/L of medium for aquatic organisms; from 2200 to 31,000 ug/kg body weight (acute oral) and 4000 to 40,000 ug/kg (dietary) for birds; and from 100 to 500 ug/kg body weight (daily dose) and 1000 to 5000 ug/kg diet for mammals. Significant adverse sublethal effects were observed among selected aquatic species at water concentrations of 0.03 to 0.1 ug Hg/L. For some birds, adverse effects--mainly on reproduction--have been associated with total mercury concentrations (in ug/kg fresh weight) of 5000 in feather, 900 in egg, and 50 to 100 in diet; and with daily intakes of 640 ug/kg body weight. Sensitive nonhuman mammals showed significant adverse effects of mercury when daily intakes were 250 ug/kg body weight, when dietary levels were 1100 ug/kg, or when tissue concentrations exceeded 1100 ug/kg. Proposed mercury criteria for protection of aquatic life range from 0.012 ug/L for freshwater life to 0.025 ug/L for marine life; for birds, less than 100 ug/kg diet fresh weight; and for small mammals, less than 1100 ug/kg fresh weight diet. All of these proposed criteria provide, at best, minimal protection.