[en] With the increasing proportion of base load nuclear power the dynamic behavior of light water reactor power stations in system operation assumes greater importance in individual electricity supply undertakings. First the paper indicates and, with the aid of the results of measurements substantiates the requirements established from the system operation point of view for various control duties (load following operation, frequency support). Then the possibilities and limiting factors of large nuclear units in operation and under construction are considered with special reference to recent findings which restrict power cycling capability (for example fuel element behaviour, life consumption in the turbine area). In conclusion the load flexibility characteristics of conventional and nuclear units are compared. (orig.)