In 1978, EPA promulgated a final rule for the control of sulfur oxide emissions at existing nonferrous smelters in Arizona. Emission limits, determined from a 1973-74 data base, were set by proportionally rolling back the 24-hourly average smelter emissions estimated by sulfur balance or other methods for the day on which the maximum three-hour-average ambient concentration of sulfur dioxide was observed. In response to the EPA rule, an alternative approach to calculating copper smelter emission limits based on a statistical model was proposed by Arizona. EPA contracted with SRI International for an evaluation of both rules. The analysis suggests that emission limits imposed by the current EPA rule provide for a greater confidence in achieving ambient air quality standards than do the limits suggested in the report by Arizona; and that the feasibility of implementing the multipoint rollback method at a smelter to produce a specific enforceable distribution of emissions has yet to be demonstrated.