Abstract: In a tailgate assembly for a dump box of the type comprising a front inner section and a rear outer section adapted for telescopic movement one with respect to the other with the inner section slidably engaging an interior surface of the outer section. The relative position of the inner and outer sections between an expanded and a collapsed position thereof is controlled by hydraulic cylinders. The tailgate assembly comprises a tailgate hingedly mounted to the rear of the outer section. A push rod has a front end thereof fixedly mounted at a front side of the inner section of the dump box. The push rod extends rearwards from its front end through a guide mounted on a sidewall of the outer section and towards the tailgate. A chain assembly connects a lower free end of the tailgate with a rear end of the push rod. The chain assembly is adapted to be in a substantially extended position thereof when the dump box is in its expanded position.