Since the discovery of substances which excite a minimal reaction in the body such as the silicones and plastics, there have been many attempts to develop artifical devices to replace diseased organs, temporarily or permanently. The dialyzers and pump-oxygenators are well known. Temporary cardiac substitution utilizing the subject's own lung for oxygenation has been carried out successfully by a number of investigators.2,4,5,10,11 Pumps intended for more permanent cardiac replacement have been made by Kolff, Akutsu and associates,1, 6 McCabe,9 Liotta et al,8 and by Kusserow.7 The authors started investigating this problem in 1957, with the feeling that the first challenge requiring solution was the development of a two-chambered device which would always pump equal quantities of blood to the pulmonary and systemic circuits. It was hoped that such a machine would not produce the pulmonary edema which resulted from earlier attempts to replace both chambers