One of the most rapidly growing human activities in the U.S. of importance to atmospheric ozone is the use of highway motor vehicles. Transportation sources are estimated to have been responsible for about 34% of 1985 U.S. anthropogenic hydrocarbon emissions, 70% of carbon monoxide emissions, 45% of nitrogen oxide emissions, 24% of nonaerosol chlorofluorocarbon emissions, and 14% of carbon dioxide emissions. Data is presented describing possible uninventoried transportation hydrocarbon emissions that could increase their estimated contribution to 45 - 50% of the anthropogenic total. Data is also presented suggesting motor vehicles to be relatively insignificant sources of anthropogenic nitrous oxide, but noting that these emissions are increased by the control technologies used to reduce hydrocarbon, carbon monoxide, and nitrogen oxides emissions. The sensitivity of motor vehicle emission rates and compositions to such operating variables as ambient temperature, altitude, and average speed is discussed.