Abstract: A hydrostatic drain plug/outlet assembly comprises a hollow body (1) with an inlet port and an outlet port for connection to a fluid conduit. A slidable valve member (6) is movable between a locked open position and a locked closed position via an intermediate position at which the valve member (6) is urged into sealing engagement with a sealing member (3) by a spring (21) which supports a hydrostatic pressure of predetermined magnitude beyond which excess hydrostatic pressure is relieved by the valve member to prevent fluid overflow in a vessel. The valve member is moved between the locked open position and the locked closed position by rotation of valve member (6).