There is disclosed herein a generator control system for use in a power supply system for vehicles in which a battery and an engine-driven generator are connected in parallel with a load. The total load current fed to the load is detected and the generated voltage level is changed by performing field regulation for the generator in accordance with the detected value. Switching of the generated voltage level of the generator is controlled according to the state of the load applied so as to improve power generation efficiency. Certain parameters, such as battery state, engine speed, auto choke and load state, can be provided as an input or inputs to a voltage regulator for the generator.