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1. Automaticity and responsiveness to various pharmacological agents were examined in isolated muscular strips from the outer wall of the left auricle of mammalian hearts. Simultaneous mechanical and electrical recording was often employed. 2. Of 65 preparations studied, 7 beat spontaneously; the others remained quiescent except for occasional isolated contractions. Fifteen were histologically examined (Professor Aschoff); none of them, even those which contracted rhythmically spontaneously, contained specific nodal tissue. 3. The strips responded to single condenser discharges or to continued faradic stimulation. The response sometimes outlasted the period of stimulation. 4. Adrenaline increased the number and force of the irregular contractions and sometimes produced rhythmic activity. Previous treatment with adrenaline facilitated the setting up of regular rhythmic response to single electrical stimuli. 5. Histamine in larger initial doses often, but not invariably, produced rhythmic activity. 6. Barium invariably produced rhythmic activity. Calcium abolished spontaneous contractions and prevented the rhythmic response to histamine. 7. Strophanthin did not produce rhythmic responses. 8. Veratrine and aconitine always produced rhythmic activity of high frequency. 9. In man, under pathological conditions, the left auricle may possibly initiate single auricular extra‐systoles and maintain rhythmic activity. We express our thanks to Professor Samson Wright for his help in preparing this paper for the press.

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