This is an experimental study of the effects of pressure on the heart. Its purpose is to determine the feasibility of decompression of a massively hypertrophied heart in the human being. It would seem that in cases in which the heart is greatly enlarged, there might be an abnormally high pressure on the heart because it is encased in the non-yielding, bony thoracic cage. In considering this question, we desire to focus attention particularly on the question of the harm produced on the heart muscle by pressure exerted on it by the wall of the chest because of the massive enlargement of the organ. We are here not considering the questions of giving relief by decortication of a thickened pericardium (Delorme) or by allowing the heart to contract against a yielding instead of a rigid structure, as is proposed in the cardiolysis of Brauer in cases of chronic mediastinopericarditis. There