Abstract: A meter prover is provided in which a substantially spherical plug travels through a U-shaped calibration conduit between two delivery/receiving stations at the ends of the conduit, and each said station contains means for selectively retaining the plug in the station. The retaining means consists of a sleeve with the same cross-section as the conduit which is mounted for horizontal movement toward and away from the conduit end along a plug cage that receives and guides the plug when it emerges from the conduit end. When the sleeve is in a position where it forms a continuation of the conduit end, fluid pressure is able to force the plug through the sleeve into the conduit, but when the sleeve and plug are spaced from the conduit end the fluid is able to bypass the plug and flow directly into the conduit.