In a device for actuating a security device, preferably for securing a motor vehicle against unauthorized use, in which a control unit transmits a first coded electromagnetic signal (stimulus signal), in which a portable transmitter (radio key) receives the stimulus signal and transmits a second coded signal (enable signal), and in which the control unit is connected to the security device and actuates the latter if the enable signal is received and recognized, both the control unit and the radio key can after the carrier frequency of the coded electromagnetic signals. During signal transmission, this frequency is altered in a manner which is known only to the control unit and to the radio key.