Abstract: A valved cannula assembly having a gasket retaining collar positioned internally within a gasket receiving member, the gasket retaining collar having a recess to receive and retain in a fixed position an annular gasket, which acts in conjunction with either a flapper valve assembly or an inserted instrument to seal the axial bore of the cannula and prevent escape of fluids or gases. A plugging member forces the retaining collar against the gasket receiving member. The recess of the gasket retaining collar is preferably provided with annular ridges to secure the gasket and a conical shoulder to allow some deflection.