Practical implementation of a fundamentally new and efficient method for optimizing the operating mode of a power system containing hydraulic and thermal power plants is presented.Calculations for optimizing the power system operating mode on an eighthour time interval are carried out taking as an example a hypothetic al power system consisting of one thermal power plant and two hydraulic power plants (which are prototypes of real hydraulic power plants). According to the present edstatement of theproblem, anintegral constraintis imposed on the amount of water spentat the hydraulic power plantsin the period for which the operating mode is optimized, the change of water head at hydraulic power plants is taken into account, and the optimal composition of operating hydraulic power units is selected at each hour in the course of calculation. The calculation is finished with determining the technicaleconomic indicators. The calculation iscompleted within 10 s. The effectiveness of the results obtained from using the proposed method and problem statement is compared with one of possible versions of simplified problem statement and corresponding procedure for solving this problem.