The research program on which this paper is based included both laboratory emission measurements and extrapolation of results to the national population of heavy-duty farm, construction, and industrial engines. Emission tests were made on four gasoline engines and eight diesel engines typical of those used in F, C, and I equipment. Gaseous and particulate emissions were measured during engine operation on well-accepted steady-state procedures, and diesel smoke was measured during both steady-state conditions and the Federal smoke test cycle.Emissions measured were hydrocarbons, CO, CO2, NO, NOx, O2, aliphatic aldehydes, light hydrocarbons, particulate, and smoke. Emission of sulfur oxides (SOx) was estimated on the basis of fuel consumed, and both evaporative and blowby hydrocarbons were also estimated where applicable (gasoline engines only).Data on emissions obtained from this study were compared with those available in the literature, where possible. Other data on equipment size, usage, and population were used with the emissions data to estimate national emissions impact. Emission factors and impact estimates were made separately for each application category (farm, construction, industrial), resulting in the conclusion that these application categories contribute a small, but significant, portion of national pollutant totals.