This paper describes design features and construction details of Ford Motor Company's recently commissioned Mobile Emissions Test Laboratory (METL). The lab contains all facilities - chassis dynamometer, gas analyzers, test environment, and support services-necessary for the conduct of federal emissions tests anywhere in the country.The METL is contained within a large semitrailer, compartmented into three sections for instrumentation, vehicle test, and support equipment. The center vehicle test section is expanded on site, providing an enclosed 13 X 30 ft area with built-in chassis dynamometer. A convenient feature is the “drive-through” for test cars. METL can operate at temperatures as low as -10 F and has provided accurate, reliable results on field test programs.