This study analyzes the local regulatory and non-regulatory determinants of ambient air quality in Allegheny, Baltimore, and Cuyahoga Counties over the period 1972-1992. Mandated pollution control investments appear to have often had a statistically significant effect in reducing maximum concentrations of suspended particulates and tropospheric ozone in these areas. The effects of regulatory air quality controls, however, generally have been overshadowed by the impacts of non-regulatory factors. In general, local regulatory and nonregulatory factors failed to account for a majority of the variation in local air quality. This underscores the importance of regional or national factors in determining local air quality.