The U.S. EPA determines emission level of automobiles through testing to estimate vehicle fleet emissions which are often critical to local air quality planning. It is important therefore, that the test procedure for determining vehicle emissions simulates, as accurately as possible, the conditions existing in areas where vehicle emissions are of major concern. The purpose of this study was to generate preliminary data on the effect of the following test variables on CO emission test results: (1) Alternate soak times prior to the '75 Federal Test Procedure (2) Alternate soak temperature prior to the '75 Federal Test Procedure (3) Alternate driving cycle. The directionality of these effects was known from previous studies. In question was the relative magnitude of each on emission levels of recent production cars as well as pre-emission control cars.