The impact of farm and industrial tractors on the total national emissions picture is difficult to assess because the great diversity of uses for these machines results in a wide variation in duty cycles and degrees of utilization. Also, there is a general lack of detailed information on population and population distribution.This paper compares possible farm tractor duty cycle models and concludes that the widely used 13-mode federal heavy-duty diesel engine emissions test cycle can be used to estimate wheel tractor emissions.Estimates then are given for the amount of pollution released by wheeled farm and industrial tractors in 1972, based on the 13-mode cycle and population estimates derived from U.S. Dept. of Commerce, U.S. Dept. of Agriculture, and Farm and Industrial Equipment Institute data. Calculations show wheeled tractor emissions contribute 3.2% or less of the national total, depending upon the pollutant species. Estimates of the emissions contribution of wheeled tractors in concentrated urban areas in California and Illinois show an even lower effect, constituting 0.6% or less of the total mobile source emissions.