Exhaust emissions from two diesel passenger cars were measured as a function of ambient temperature (43 - 82°F) over the urban dynamometer driving schedule of the Federal Test Procedure (FTP). Low ambient temperature did not affect rates of emission of total hydrocarbons, carbon monoxide, or nitrogen oxides. Fuel economy decreased with decreasing test temperature. Total particulate matter increased with decreasing test temperature, as did particulate organic emission rates (due primarily to adsorption of uncombusted diesel fuel). Polynuclear aromatic hydrocarbon emissions (ug/mile) and mutagenicity in the Ames Salmonella bioassay (rev/mile) were not affected by decreasing test temperature.