Abstract: A motor-pump unit for submerging down a well, e.g. an oil well, comprises a motor portion driving a pump portion. The motor portion is filled with fluid to prevent ingress of the surrounding medium which the pump portion pumps. The fluid is maintained at a higher pressure than the surrounding medium by including a flexible tank filled with fluid and in communication with the fluid in the motor portion. The flexible tank is compressed both by the surrounding medium and by resilient compression means so that the pressure inside the motor is higher than the surrounding pressure, thereby ensuring any leakage is outwardly from the motor. Down the well the temperature rises causing the fluid to expand, so the flexible tank is left substantially empty when the unit is prepared at the surface prior to lowering.