Three production cars and seven cars with various experimental exhaust emission control systems were tested to determine the effect of winter weather ambient temperatures on exhaust emissions. All of the cars were tested at 70, 40, and 20°F, and one was tested at 0°F using the 1972 EPA test schedule. Modal and bag emission data as well as catalytic converter temperature data were obtained. The results of these tests are presented in this paper. There was an increase in HC and CO emissions as ambient temperature was reduced. Most of the increase came from the first cycle of the 18 cycle test and was the result of additional time required to reach operating temperatures. NOx emissions did not vary substantially with ambient temperature.