During commissurotomy, performed on a patient with rheumatic mitral stenosis and chronic impure auricular flutter fibrillation, it was possible to do a direct ecg. exploration of the operatively exposed left auricle and its appendage; either one standard lead and one semi-unipolar direct lead, or 2 semi-unipolar direct leads from different sites of the epicardium, were taken. At different sites of the auricle and its appendage, the ecg. varied greatly as follows: either rapid, or sharply pointed, or slow waves, or scarcely visible, or even reduplicated P waves were recorded; their time of appearance differed at the various sites[long dash]asynchronously [long dash]but the P-P intervals, at all areas, were fairly identical[long dash]isorhythmic. At sites farther distant from the venae cavae, the P waves occurred earlier than at sites nearer to the cavae. These first direct investigations on fluttering human auricles suggested that polyfocal stimulation must be present in auricular flutter. There is great diversity in the excitability of the various portions of the auricular wall and its appendage, with minimal excitability at some areas, and the stimuli do not occur simultaneously, while the refractory period for every portion of the wall is practically identical. In the reduplicated P waves, seen at different sites, one wave is "intrinsic", the other "extrinsic", and the latter may be induced earlier or later than the "intrinsic" wave.