The closed system connector assembly is formed with a male connector and a female connector. Both connectors have rubber membranes which can be pierced by a hollow needle disposed within the female connector. In some embodiments, the membrane of the female connector is mounted on a collapsible tube disposed about the hollow needle. During insertion of the male connector, this tube collapses and, upon withdrawal, the tube expands. In another embodiment, the female connector has a housing which carries the needle and is slidable within a sleeve which carries the membrane.