Rock preconditioning in advance of mining has been an effective means of rock-burst control based on small-scale field demonstrations. The objective of this investigation was to test the effectiveness of rock preconditioning on a larger scale and to determine whether the preconditioning would be effective over a specific period of time. Preconditioning in 8 and 12 crosscuts on the 7900 level of the Hecla Mining Company's Star Mine involved about 7,232 feet of drilling and the blasting of about 10,055 pounds of high explosives to precondition a block of ground along the main vein of about 450 feet long, 50 feet above, and 40 feet below the level. The slight amount of seismic energy accompanying the initial raiseup in the preconditioned stopes indicates that this preconditioning was successful. Continued monitoring should be carried out to determine whether the preconditioning remains effective over the length of time required for the completion of mining in a stope block.