Riparian wetlands contaminated with Hg from an industrial point source were found to be important sites of production and release of methyl mercury (MeHg) in a 40-km reach of the Sudbury River in eastern Massachusetts. Stream discharge and concentration measurements were used to calculate annual mean loads for total Hg (??Hg) and MeHg in contaminated river reaches, a reservoir, and a riparian wetland downstream from the industrial source. Budgets based on these loads indicate that the annual mean ??Hg load increased sixfold in a reach receiving flow from the point source, but the annual mean MeHg load did not increase. About 23% of the ??Hg load was removed by sedimentation during flow through the reservoir. Net production of MeHg in the reservoir was similar to that reported elsewhere for lakes receiving Hg from atmospheric deposition only. ??Hg concentrations and loads increased significantly as the river passed through the riparian wetland reach. On the basis of flooded wetland area, net production of MeHg was 15 times greater in the wetland reach than in wetland-associated drainages described in other studies.