A portable data exchange device including a data input, a data output, and a memory is incorporated into a mechanical or electronic key that is required for operating a motor vehicle. The data exchange device can exchange data bi-directionally or uni-directionally with a data processing device in the vehicle and with an external input/output terminal. The data may include vehicle-specific and/or driver-specific data relating to various systems of the vehicle. The data are transmitted to and from the device in the key in a wireless or direct-contacting manner. This device incorporated in the key avoids the need for carrying a separate data unit for exchanging data between the on-board data processing device of the vehicle and the external input/output terminal.