INTRODUCTION A national program to control exhaust emissions from automobiles generates a requirement for surveillance activities having several objectives. ? Predicting the total emissions to the atmosphere in various cities and/or air pollution control regions as a step toward establishing air quality goals and attendant emission standards. ? Comparing various control alternatives such as programs of mandatory vehicle inspection and maintenance by directly contrasting emissions between regions in which alternate procedures have been implemented. ? Examining the effectiveness of control equipment installed on automobiles both for its performance when new and also to establish the nature and extent of its performance degradation when used by the general public.