Measurements of TotHg (total mercury) and MeHg (methylmercury) in runoff from the covered catchment G1 and the reference catchment F1 at Lake Gårdsjön, Sweden, have been performed and evaluated. The roof over the covered catchment limits atmospheric deposition input of TotHg and MeHg and a response in runoff concentrations and transport was expected. Based on data from 10 yr of monitoring, no statistically significant change in runoff flux of TotHg or MeHg can be observed. A slight decrease in MeHg output in the covered catchment was observed after 2 yr of the experiment. This can be explained as a temporary effect caused by the roof construction. The main conclusion is that release of TotHg and MeHg from the forest soil is controlled by factors other that wet deposition input, for example the mineralisation of organic matter. Furthermore, there is no indication of a depletion of the Hg pool in the soil. In spring 1999, the reference catchment F1 was affected by forestry machinery significantly disturbing the forest soil layer in a limited area. This caused MeHg concentrations to increase dramatically in runoff and led to an increase of the annual transport by at least a factor of 3. This indicates that forestry and other activities that disturb forest soils may be important for controlling MeHg fluxes to aquatic ecosystems.