The overall objective of the program was to provide vehicle emission data to be used to establish and predict vehicle emission source data on a metropolitan and nation-wide basis. A laboratory study was conducted on 1966 through 1972 model-year vehicles operating in three U.S. metropolitan areas, Denver, Houston, and Chicago. The three cities study was a part of a larger study involving emission testing in six U.S. cities. Five-hundred and ten vehicles, one-hundred and seventy in each city, were tested to determine exhaust hydrocarbon, carbon monoxide, carbon dioxide and oxides of nitrogen emissions. A subsample of twenty of the 1972 model-year vehicles in the Denver area were also tested for evaporative hydrocarbon losses. Exhaust emission tests were conducted according to the 1975 Federal Test Procedures on vehicles as received.