Abstract: A fast closing valve for gaseous and liquid medium closes, against a resilient force, upon the occurrence of a sudden pressure difference in the conduit, specially aeration conduits for use in anti-aircraft protection shelters. The main part is a valve-flap having the form of a dihedron (10), which is mounted, pivoting easily within a chamber (9) about the axis of the vertex of the dihedron (axis 11). On either side of the valve-flap (10a, 10b) are associated deviation openings (7,8) of the chamber (9) which will be closed alternately by the pivoting of the flap (10). The flap (10) is normally held in a stable open position by a spring arrangement (14) until one or the other of the flap sides is subject to a pressure wave causing the closure of one or the other of the openings (7,8).