Mercury (Hg) concentrations in burbot (Lota lota), perch (Perca fluviatilis), northern pike (Esox lucius), roach (Rutilus rutilus), whitefish (Coregonus lavaretus) and peled (C. peled) were monitored in 18 reservoirs situated in western and northern Finland over a period of 16 years (1979-1994). The reservoirs were impounded from 1964 to 1980. The surface areas ranged between 1 and 417 km2. Data from downstream sites and reference lakes were compared to reservoir mercury data. Generally, fish Hg concentrations in reservoirs exceeded those in natural lakes. Shortly after inundation, fish mercury concentrations clearly increased and remain 15-25 years above background concentrations. The mean Hg concentrations in standardized 1 kg pike from 1989 to 1993 was 0.45 mg/kg in natural lakes and 0.58 mg/kg in reservoirs. The 1 mg/kg fish Hg level recommended by health authorities as the upper limit for human consumption was still exceeded in two reservoirs 20 years old or more. The two largest reservoirs (24 and 27 years old in 1994) with significant commercial fishing had Hg levels below 0.5 mg/kg in all fish species studied. Statistical models for predicting the Hg concentrations in the weight standardized pike were verified with the observed data. For most of the reservoirs, predictions from models based on pH, organic matter (COD) in water, the extent of water level regulation and reservoir age agreed well with observed data. In the most southern, new (under 20 years old) and heavily regulated reservoirs, pike Hg concentration followed a logarithmic function of reservoir age. Model predictions of mercury in pike in the planned Vuotos reservoir (in northern Finland) predicted that Hg would exceed 1 mg/kg Hg for the first 12 years after the flooding.