Isolated pairs of rabbit auricles have been observed in media containing 6, 24 or 75 mm-potassium, with corresponding reductions in sodium concentration. In 24 mm-potassium, adrenaline restored beating and excitability, as did calcium chloride, but ouabain had no effect. In 75 mm-potassium, adrenaline had no effect; calcium chloride caused a contracture; ouabain had no direct effect, but auricles which had been beating in the presence of ouabain contracted promptly on transfer to 75 mm-potassium. Left auricles, which do not beat spontaneously, were less sensitive to calcium and to ouabain. The results showed a membrane stabilizing action of calcium and an action on muscular contraction, and suggested that cardiac glycosides acted by causing accumulation of calcium at the activator site in the tissue.