An implantable cardiac stimulation device, mode switchable between atrial tracking and atrial non-tracking pacing modes includes a threshold control for controlling atrial sensing threshold. The threshold control reduces the sensing threshold from a first threshold used during the atrial tracking pacing mode to a second threshold between the first threshold and the noise for use during the atrial non-tracking pacing mode. The necessity for readjusting the atrial sensing threshold is based upon the ratio of the sensing threshold and atrial amplitudes or upon a diagnostic distribution of atrial amplitudes.