Experimental catalysts for the reduction of oxides of nitrogen (NOx) were evaluated on a 258-horsepower (192 kW) direct-injection heavy-duty diesel engine. An experimental reductant delivery system provided supplementary hydrocarbons for the reduction of NOx. Initially, diesel fuel was used as the supplementary reductant. Early experiments resulted in a 10 to 17 percent reduction in NOx emissions when tested using the heavy-duty engine transient Federal Test Procedure (FTP), and a 30 to 40 percent reduction at selected steady-state catalyst inlet temperatures. A fuel economy penalty of five percent was measured for initial FTP experiments. Emissions of total hydrocarbons (HC), carbon monoxide (CO), and particulate matter (PM) tended to increase during initial experiments with the addition of the supplemental reductant, but these emissions decreased with the incorporation of improved catalyst formulations and reductant fuel spray calibrations. Additional experiments were performed with ethanol and toluene as supplemental NOx reducing agents. Emissions of nitrous oxide (N2O) were measured and found to increase when NOx emissions were reduced. Steady-state emissions tests revealed a very narrow temperature window for NOx reduction. Initial project results are encouraging, but further catalyst and system development is required to meet future emissions performance and durability requirements.