A longitudinal rail guide of a motor vehicle seat having a bottom rail, with a top rail which is movable in longitudinal direction relative to the bottom rail and with a locking device. The locking device has a guide part which is disposed on the top rail and has at least one guide opening, and for each guide opening there is a blocking part which is disposed so as to be movable in the guide opening. The guide part has a free edge which lies directly opposite an adjacent inner surface of the bottom rail, and before an accident the free edge is located at a small distance from the adjacent inner surface of the bottom rail and after an accident the free edge bears against the adjacent inner surface.