Abstract: An on-off valve for medical procedures and laboratory devices, that has particular use with endoscopic medical procedures. Also featured are endoscopic devices that comprise such valves. The valve preferably includes a valve body, a valve member, a flexible seal member, a compression mechanism and a rotating mechanism. The body includes a centrally disposed cavity and two through apertures communicating therewith. The flexible seal member includes an internal chamber and two through flow apertures. The internal chamber is configured to rotatably receive the portion of the valve member including the internal flow passage. The seal member is mounted on the valve member and the seal/valve member assemblage is disposed in the body cavity. The compression mechanism acts on a surface of the seal member after it is disposed in the seal cavity so the seal member sealingly engages opposing surfaces of the valve member and the body.