In the context of examination of the projected hydropower station on the Katun river and placing it in the so-called Sarasin-Kurai mercury-bearing zone, a team of experts of the Siberian Division of the Russian Academy of Sciences have conducted all-round research. The monograph is devoted to the geochemical, hydrogeological and hydrogeochemical data obtained from analysis of a great body of factual material. Hundreds samples of rocks, bottom sediments, soils, underground and surface waters have been taken up and screened for mercury. Mercury has been found to be distributed very unevenly in the rocks of ancient lithocomplexes: from 2 to 115000 ppb. The local background is higher (100 ppb) than the clarke (454 ppb). In bottom sediments, the mercury content varies between 2 and 3600 ppb, whereas the background content (30-150 ppb) is 3 and more times less than its clarke for mudrocks. Higher mercury concentrations are typical of mud fractions less than 0.005 ml. The mercury content in soils has been assessed as a few tens of milligrams per ton. It is found mainly in A1 and A2 horizons. In underground waters (filtrate), samples taken from wells, mercury content varies in the range of 0.01-1.26 mkg/l, the average being 0.02-0.03 mkg/l; in springs - 0.03-0.35 mkg/l, the average being 0.04 mkg/l. It has been demonstrated that the inflow of underground waters where reservoirs are scheduled for construction will be 0.2-0.4 km(3) per annum. The most likely underground release of mercury is estimated as 0.05-0.013 ton per annum. Studies of the mercury balance in the "water-rock" system allow the authors to tentatively assume that no considerable redistribution of mercury in the underground waters will take place after building of the Katun reservoir.