A percussion blow added manually operable drilling tool, such as a hammerdrill, has a housing (2) containing an electric motor for rotating a tool bit chuck (9) about its axis, a control device (C) for controlling the rpm of the chuck (9) operated by the electric motor (3) which is activated by a switch (5) located on the exterior of the housing (2), and a percussion blow mechanism for periodically applying axially directed blows to a bit (10) secured in the chuck (9). The electric motor (3) is coupled to a device (12) for measuring the reaction torque. The control device (C) is connected to an evaluation unit (E) which is connected to the device (12) for measuring the reaction torque so that the rpm of the chuck can be regulated from maintaining the measured reaction torque at a maximum.