||A report is presented of a program for which the objective was to reduce NO(x) mass emissions from a 4000-pound GVW vehicle by extending the operation of the engine in the ultra-lean air/fuel regime. The Phase I effort of the program consisted of a baseline vehicle evaluation with a carburetor, installation of Electronic Fuel Injection (EFI), and the baseline evaluation with EFI. The Phase II effort consisted of removing the engine from the vehicle and installing it on an engine dynamometer, performing steady-state tests at selected power test points, and, finally, exploring the possible ultra-lean operation using various parameters. The potential of reducing the NO(x) emissions with ultra-lean mixtures is seen clearly. However, ultra-lean mixtures are very prone to misfire resulting in increased HC emissions and rough engine operation giving poor drivability. On the other hand, the oxygen-rich exhaust, resulting from lean operation, appears to offer the possibility of a thermal reactor or a catalyst for oxidizing HC without secondary air.