Method for operating a brake system for motor vehicles, which in a "brake-by-wire" operating mode can be controlled both by the vehicle driver and independently of the vehicle driver, which is preferably operated in the "brake-by-wire" operating mode and which can be operated in a first reversion operating mode, in which a brake booster is available, and in a second reversion operating mode without brake booster, in which only the operation by the vehicle driver is possible, comprising a) at least two piston-cylinder arrangements (3a-d) for generating brake pressures in wheel brakes of the vehicle, b) an actuating unit operatively connected to the piston-cylinder arrangements (3a-d) on the inlet side for their mechanical actuation in a second reversion operating mode, c) a brake pedal connected to the actuating unit on the inlet side for control of the brake system by the vehicle driver, d) a hydraulic circuit, which is operatively connected to the piston-cylinder arrangements (3a-d) on the inlet side and which substantially comprises an electrically controllable, hydraulic pressure source, a high-pressure accumulator that can be charged by the pressure source for maintaining hydraulic servo power, and electrically actuated pressure-regulating valve devices (4, 5), which comprise a first connection or an outlet connection for hydraulic control of the piston-cylinder arrangements (3a-d), a second connection, which is connected by an electrical actuation to the first connection, and a third connection, which in the unactuated state of the pressure-regulating valve devices (4, 5) is connected to the first connection and which can be shut off in a through-flow direction by an electrical actuation, e) a pressure-regulating valve arrangement, which can be actuated by the brake pedal and which in the first reversion operating mode provides a brake system pressure, proportionable via the brake pedal, f) a driver intention registering device, g) a hydraulic fluid reservoir, which is assigned to the actuating unit and/or to the piston-cylinder arrangements (3a-d), and h) an electrical power supply (26, 27) together with i) an electronic control unit, characterized in that the "brake-by-wire" operating mode intended as normal operating mode is divided into an energy-saving operating mode and at least a first high-power operating mode, between which switching occurs as a function of the braking situation and which utilize different components of the hydraulic circuit, wherein in the energy-saving operating mode the pressure provided by the electrically controllable pressure source is delivered to the third connections of the pressure-regulating valve devices (4, 5), whilst the pressure maintained by the high-pressure accumulator and present at the second connections of the pressure-regulating valve devices (4, 5) remains unused, and wherein in the first high-power operating mode the pressure supplied by the high-pressure accumulator is delivered to the second connections of the pressure-regulating valve devices (4.5). a) at least two piston-cylinder arrangements (3a-d) for generating brake pressures in wheel brakes of the vehicle, b) an actuating unit operatively connected to the piston-cylinder arrangements (3a-d) on the inlet side for their mechanical actuation in a second reversion operating mode, c) a brake pedal connected to the actuating unit on the inlet side for control of the brake system by the vehicle driver, d) a hydraulic circuit, which is operatively connected to the piston-cylinder arrangements (3a-d) on the inlet side and which substantially comprises an electrically controllable, hydraulic pressure source, a high-pressure accumulator that can be charged by the pressure source for maintaining hydraulic servo power, and electrically actuated pressure-regulating valve devices (4, 5), which comprise a first connection or an outlet connection for hydraulic control of the piston-cylinder arrangements (3a-d), a second connection, which is connected by an electrical actuation to the first connection, and a third connection, which in the unactuated state of the pressure-regulating valve devices (4, 5) is connected to the first connection and which can be shut off in a through-flow direction by an electrical actuation, e) a pressure-regulating valve arrangement, which can be actuated by the brake pedal and which in the first reversion operating mode provides a brake system pressure, proportionable via the brake pedal, f) a driver intention registering device, g) a hydraulic fluid reservoir, which is assigned to the actuating unit and/or to the piston-cylinder arrangements (3a-d), and h) an electrical power supply (26, 27) together with i) an electronic control unit, characterized in that the "brake-by-wire" operating mode intended as normal operating mode is divided into an energy-saving operating mode and at least a first high-power operating mode, between which switching occurs as a function of the braking situation and which utilize different components of the hydraulic circuit, wherein in the energy-saving operating mode the pressure provided by the electrically controllable pressure source is delivered to the third connections of the pressure-regulating valve devices (4, 5), whilst the pressure maintained by the high-pressure accumulator and present at the second connections of the pressure-regulating valve devices (4, 5) remains unused, and wherein in the first high-power operating mode the pressure supplied by the high-pressure accumulator is delivered to the second connections of the pressure-regulating valve devices (4.5).