1. A method is described for determining the refractory period of the dog's auricle during the normal sinus rhythm. The advantages of the method are:
(a) The total stimulating effects of repeated induction shocks are avoided.
(b) The action current is recorded from a point one millimeter or less from the point of stimulation.
(c) Alterations in the spontaneous rate of the auricle do not interfere with the accurate determination of the refractory period.
2. The values obtained for the normal refractory period and the changes produced by atropine and by stimulation of the vagus agree closely with those of previous observers.
3. The automatic features of the method make possible the determination of the refractory period under adrenalin. This drug brings about a distinct shortening of the refractory period but less than that produced by stimulation of the vagus.
4. During vagal stimulation a single induction shock, introduced soon after the end of the refractory period, frequently produces auricular fibrillation. The cause of this irregularity is discussed and its relation to clinical auricular fibrillation is suggested.