A predispatch model that minimizes generation and transmission losses on hydroelectric power systems is presented and applied to the hydroelectric power system of COPEL, a Brazilian utility located in southern Brazil, Power loss in the generation and transmission systems is minimized on an hourly basis throughout a day, Load demand as well as generation, transmission and interchange operational constraints are satisfied. Hydroelectric generation characteristics are described in detail. Power loss in hydroelectric generation is associated with a reduction in turbine-generator efficiency and effective water head, The latter is due to an increase in tailrace elevation and penstock head loss, Power loss upon transmission is calculated as a quadratic function of active power now, as represented in a DC load flow model, The predispatch model, was evaluated under typical operational conditions, achieving significant savings with respect to actual operation.