Studies on the action of acetylcholine on the sino-auricular node of the dog.

1. The response of the sino-auricular node to direct injection of acetylcholine in the artery supplying the node has been studied in the dog.2. Acetylcholine in graded doses (2, 5, 10 mug) caused acceleration of the sinus rhythm.3. The time course of the positive chronotropic action of acetylcholine resembled that produced by adrenaline or noradrenaline.4. The positive chronotropic action produced by acetylcholine was blocked by dichloro-isopropylnoradrenaline and pronethalol and potentiated by cocaine.5. In reserpinized dogs, acetylcholine in similar doses produced sinus bradycardia. The positive chronotropic response could be temporarily restored by infusion of adrenaline or noradrenaline.6. Atropine blocked both the positive and negative chronotropic actions of acetylcholine.