Concentrations and fluxes of greenhouse gases methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) were measured during open water conditions in two hydroelectric reservoirs, Lokka and Porttipahta, in the northern boreal zone in Finland. These reservoirs were located on peat and forest soils and were built in 1967 and 1970, respectively. Over 20 years after their flooding, the reservoirs were still largely supersaturated with dissolved CH4 and CO2. Measured with floating static chambers, the stations in Lokka released more CH4 (means of 5.3--119 mg m-2 d-1) during the open water periods in 1994 and 1995 than the stations in Porttipahta (means of 2.5--4.8 mg m-2 d-1), measured in 1995. The mean CO2 emission rates in Lokka (21--133 mg m-2 h-1) and Porttipahta (36--95 mg CO2 m-2 h-1) were more similar. The chamber measurements could not detect episodic CH4 ebullition in Lokka Reservoir, indicated by generally higher CH4 fluxes (means of 2.6--660 mg m-2 d-1) obtained with bubble gas collectors than with the chambers. The seasonal mean N2O fluxes were generally low, ranging from -89 to 270 ¿g m-2 d-1. There was no clear association between the CH4 emissions and the bottom type, including mineral soils and old peat deposits. The flooded vegetation, higher nutrient content, and primary production in the water column rather than old peat could account for the higher CH4 emissions from the stations in Lokka. This conclusion is supported by the high content of modern carbon (C) in methane (percent modern C of 92--104%) that was extracted from the sediment of Lokka. The results suggested that if there is a good long-term availability of phosphorus and nitrogen, the intensive internal C cycle associated with the primary production could maintain high CH4 and CO2 production for decades, similar to the situation in eutrophied natural lakes.