The heart often appears to function independently because it can maintain regular activity even when isolated from the rest of the body. Likewise, it maintains its regularity in the midst of powerful but balanced neural and pressure influences. Experimental situations designed to unbalance these forces produce rhythm disturbances resembling those encountered clinically. Unbalancing of the sympathetic supply with the parasympathetic supply totally obliterated, leads to tachycardias of various origin which are eliminated by adrenergic blockade. Removing the sinoatrial node results in atrial brady-tachyarrhythmia, eliminated by cholinergic blockade. Unbalancing the pressure load results in premature ventricular contractions that are clearly not mediated by external neural controls. Cardiac rhythm disturbances thus result from a variety of forces that may be readily tolerated when balanced by opposing forces but cannot be controlled without that buffer.