For a sample including 22 lakes, the Cs-137 concentrations in water were determined by the sorption-diffusion model. The 137Cs levels in waters of lakes of the Kola Peninsula and Karelian Isthmus are satisfactorily described by the suggested model at the distribution coefficient K-d of 4000 L kg(-1) and diffusion coefficient D of 1.0 x 10(-7) cm(2) s(-1) for 2-4-year exposure. For mid-latitude lakes, the ranges of K-d and D of Cs-137 were 6000-1200 L kg(-1) and 1.0 x 10(-7)-0.2 x 10(-7) cm(2) s(-1), respectively. The Cs-137 concentration in water is determined by the major contribution of silts, which are characterized by high K-d of Cs-137 and by large thickness of sediments on the bottom of lake hollows, to the watter-bottom sorption system. After 2-year exposure of "Chernobyl" Cs-137 in lakes, its inventory in water relative to the amount that fell out in 1986 did not exceed 14% for deep lakes and 2-3.6% for shallow lakes.