A technic for lumbar perirenal gas insufflation was presented by Rosen-stein (4) and Carelli and Sordelli (1) in 1921. In 1950, Ruiz Rivas (3) introduced his method of retroperitoneal gas insufflation by the presacral route. More recently Betoulieres et al. (2) have mentioned the introduction of the gaseous medium into the subpubic space. The presacral technic described by Rivas appears to be the method most commonly employed today when retroperitoneal gas study is desired. The essential feature of the Rivas technic is the insufflation of gas into the retroperitoneal tissues through a needle introduced into the presacral areolar tissue at any of several points on the skin adjacent to the bony coccyx. It is established practice for the operator to maintain an index finger in the patient's rectum during the procedure to insure proper retrorectal location of the needle. The purpose of the present article is to present a modification of the Rivas technic, introduced by Dr. C. J. Hodson and in use at Universit...