A procedure has been developed to estimate surface water concentrations of toxicants (“wildlife values”) that will protect the viability of wildlife populations associated with aquatic resources. This procedure was designed primarily to protect piscivorous birds and mammals from compounds that bioaccumulate in fish and was used in the Great Lakes Water Quality Initiative (GLI) to calculate wildlife values (WV) for mercury, DDT/DDE, total polychlorinated biphenyls (PCBs), and 2,3,7,8-tetrachlorodibenzodioxin (TCDD). Published in 1995, and expressed as total mercury in unfiltered water, the final wildlife value (WVf) for mercury derived in the GLI was 1300 pg Hg/L. This value was selected as the wildlife criterion (WC) for mercury in the Great Lakes basin. A second WVf for mercury was derived in 1997 as part of a Congressionally mandated report on airborne mercury emissions. These calculations were based upon mercury speciation data that were largely unavailable when the GLI was developed. Important features of the WVf in the Report to Congress include its calculation on a dissolved methylmercury basis and a reliance on field data to estimate fish bioaccumulation factors. Calculated as methylmercury in filtered water, the WVf derived in the report is 50 pg Hg/L (equivalent to 54 pg MeHg/L). A comparison of WV in the GLI and the Report to Congress requires that average values be specified for mercury speciation in natural systems. Based on this information, the WVf given in the report corresponds to a value of 910 pg Hg/L, as total mercury in unfiltered water, or about 70% of the WVf derived in the GLI. In this article we describe the algorithm used to derive WV in the GLI and the Report to Congress and review its application to mercury. Scientific uncertainties in deriving WV, particularly as they apply to mercury, are critically examined.