Previous writers1have commented on the confusion which exists on the subject of intra-abdominal pressure. The recent literature regarding this is small, but one finds many conflicting statements. Some writers state flatly that the intra-abdominal pressure is negative; others, that it is positive, atmospheric or variable. I cannot agree with the assertion in a recent paper2that "Several facts have been well established in the literature. There is normally a sub-atmospheric pressure within the peritoneal cavity." The authors of this paper used "intraperitoneal" and "intra-abdominal" interchangeably. In succeeding paragraphs it will be shown that it is not "well established in the literature" that the intra-abdominal pressure is negative and also that careful distinction must be made between the terms just referred to. The discrepancies in the literature are due to confusion in the use of these terms and to faulty interpretation of experimental results. The situation was well