The body of information presented in this paper is directed to those individuals concerned with the effects that changing altitude has on vehicle exhaust emissions.Emission data were developed from a sample of resident vehicles in Leadville, Colo. at a nominal elevation of 10,000 ft above sea level. Emissions-concentration sampling was performed on each vehicle in the sample at loaded and unloaded engine operating conditions. Concentration data were then converted to mass data using equations developed from key-mode and mass emissions testing of vehicles operating in the Denver area. Mass emissions data were then combined with mass data developed from vehicles operating in a metropolitan area situated near sea level (Chicago) to define the emissions versus altitude relationships for exhaust hydrocarbons (HC), CO, and NOx. HC and CO emissions at idle were also examined with respect to the applicability of a statewide minimum HC/CO idle emission standard for the State of Colorado.Contrary to expectations both mass CO emissions and CO emissions at idle were found to be lower from 1967 and older model-year vehicles operating in the Leadville area than from a similar sample of vehicles operating in the Denver area. These findings would indicate that vehicles operating at the higher elevations were probably modified in an attempt to improve vehicle operating characteristics. Also, since idle CO emissions were generally lower from vehicles operating at the extremely high elevations than from vehicles operating at lower elevations, a minimum HC/CO idle emission standard appears to be impractical for statewide application.