An active implantable medical device of the AAI/DDD type, notably a cardiac pacemaker, including automatic mode adjustment at implantation. The device detects spontaneous atrial and ventricular events, and is able to pace the atrium and ventricle. The device can operate in an AAI mode with ventricular sensing, a DDD mode, and includes automatic mode commutation, able to schedule, as a function of predetermined criteria, commutation from AAI to DDD mode, and reversely from DDD to AAI. The device automatically detects an implantation and has its initial mode of operation adjusted upon detection of the implantation (10). The device temporarily operates (14) in an AAI mode with activation of the means for mode commutation, then analyzes (20) the atrio-ventricular conduction so as to detect whether a potential conduction disorder exists. In case of a potential conduction disorder existing (presence) (24), the device is then set to a DDD mode and the automatic mode commutation is inhibited. Otherwise (26), the device is set to an AAI mode, and the automatic mode commutation remains activated.