A gearwheel motor has two mutually engaged externally toothed wheels (14, 15) against the side surfaces of which is pressed a sealing plate (26) by means of two approximately concentric compression fields (A, B). The two compression fields do not communicate with each other. The pressure of one compression field (B) can be controlled by means of an electro-magnetically driven control valve (41), thus changing the compression force of the sealing plate (26) on the toothed wheels. When the gearwheel motor is started, the compression force must be low, so that the starting resistance may also be low. Under high pressures, the compression force is increased by closing the valve, avoiding oil leakage along the toothed wheels from the high pressure to the low pressure side.