Regional thickness variations of the M bed limestone in the Tri-State zinc-lead district were investigated by determining the elevations of the top and bottom of that important ore-bearing horizon in 447 selected churn drill hole logs. This bed is the principal host rock for the ore deposits in the Picher field, and it is an important ore-producing horizon in many of the other mining fields of the Tri-State district of southwestern Missouri, southeastern Kansas and northeastern Oklahoma.The M bed was deposited with a greater original thickness, about 100 to 110 feet, in the vicinity of the Oronogo-Webb City-Duenweg field, than it was to the west near the Picher field where it was deposited in thicknesses of 70 to 80 feet. Post-Keokuk erosion has caused local removal of the top of the M bed in the western portion of the Picher field, and westward from that field the bed is progressively thinned by post-Keokuk erosion.In the Picher field, the principal ore-producing field of the Tri-State district, the M bed is markedly thinned and the isopachous bands exhibit a zonal pattern centered upon the ore deposits. On the scale of this regional study, more than one-half of the principal mined area in the Picher field exhibits M bed thicknesses of less than 20 feet; nearly all of the mined area occurs where M bed thicknesses are less than 30 feet. Outside the mining field there are progressively thicker M bed zones up to a maximum of 70 to 80 feet. The Galena and Melrose mining fields exhibit similar, but less well-defined, zonal patterns of M bed thickness.The thinning of the M bed associated with the ore-producing fields has originated principally by solution of the host rock limestone. The writers believe that the solution of the limestone was associated with the post-Keokuk erosion surface, and that solution of the M bed continued during some of the subsequent periods of erosion, but that some solution of the M bed host rock limestone was accomplished by the ore fluids themselves.