Abstract: A self-sealing cannula assembly for maintaining pressure in a body cavity during insufflatory surgery. A cap made of elastomeric material is fitted to an enlarged portion of a cannula and has means for simultaneous sealing and fastening the cap to the cannula. The endwall of the cap may be apertured to permit entry and exit of a trocar or other surgical instrument without interruption of the seal. Another embodiment provides an elastomeric adaptor cap arranged to be fitted to the exterior circumference of the above-mentioned cap for temporary insertion and withdrawal of instruments of relatively small diameter without removal of the original cap.