A study was made of the heart fromOpsanus tau using an electrocardiograph to record normal activity and the responses to drugs and to experimental manipulations. Bradycardia occurs in toadfish kept out of water for more than 5 minutes. Large doses of darstine, an atropine-like drug, bring about A-V dissociation and prolongation of conduction time, probably by direct action on the heart muscle. Changes in temperature modify the heartin vitro andin situ.