Monte Carlo mass production for the ZEUS experiment on the Grid

The detector and collider upgrades for HERA-II have considerably increased the demand on computing resources for Monte Carlo production for the ZEUS experiment. In order to close the gap, the existing production system was extended to be able to use Grid resources as well. This integrated system has been successfully in production since November 2004. A total of 275 million events have been simulated by thirty different Grid sites within eight months exceeding the capacity of the old system. We present the production setup and introduce the toolkit that has been developed by ZEUS for efficient use of the existing Grid middleware.