Disclosed is a self-occluding cannula assembly comprising a cannula sheath having a connector hub formed on one end thereof. A self-actuating occluding means is formed within the cannula assembly so as to prevent back flow of body fluids from the cannula. In the cannulae wherein an introducer needle is initially axially positioned within the cannula lumen, the occluding means will permit desired withdrawal of the needle but will thereafter immediately occlude the cannula lumen to prevent back flow therethrough. The occluding means is further adapted to shift to a "non-occluded" configuration upon connection of a second infusion or monitoring tube to the proximal end of the cannula. If, during use, the infusion/monitoring tube should become disconnected, the occluding means will again shift to its "occluded" configuration so as to prevent back flow of fluids from the cannula until such time as the infusion/monitoring line is reconnected.