An improved luer lock receiving septum having a configuration which provides rapid and tight resealing and yet allows penetration of the septum by the luer tip with a low penetration force. The elongated septum includes an upper portion of enlarged diameter having a target surface, a central slit, and a central, lower septum extension projecting about the slit below the upper portion and into a housing so that following luer insertion there is provided sufficient room for both the laterally displaced extension of the septum, the luer taper, and the housing to be received into a conventional luer lock connector. The septum is further preferably configured to minimize or eliminate the negative pressure deflection normally associated with the withdrawal of the large diameter luer cannula from an enclosed fluid filled lumen or chamber, by substantially isolating the lumen or chamber from the septum material displacement resultant from luer insertion. In an exemplary embodiment, the luer receiving septum is provided at a port of a stop cock.