An aerosol size analyzing system developed at the University of Minnesota was used to measure in situ primary aerosols produced by operation of automobiles on a chassis dynamometer and secondary aerosols produced by photochemical reactions of exhaust vapors in a 610 cu ft smog chamber. The total automotive contribution to urban aerosol concentrations is estimated using the volume emission rate of primary aerosol together with the relative concentrations of primary and secondary aerosols. The size distributions and primary aerosol concentrations determined in the laboratory agreed quite well with results obtained alongside roadways using a similar aerosol analyzer. For exhaust dilution ratios of 20/1 and 40/1 in a laboratory dilution tunnel, the predominant mode in the aerosol size distributions occurred at about 0.03 μm diameter. This size is substantially smaller than that reported in other laboratory studies. The size distribution of the diluted exhaust is sensitive to the rate of dilution at the tailpipe. At lower than realistic dilution ratios, the distribution modes shift to larger aerosol diameters. Differences between the volume concentration of aerosols emitted from automobiles operating on leaded and nonleaded fuel are variable depending on vehicle operation. In general, the concentration ratio of aerosols generated in leaded and nonleaded operation ranges from 3/1 to 10/1. Rates of photochemical oxidation, as determined in a smog chamber, were similar for exhausts from leaded and nonleaded fuels. Primary aerosols from these exhausts can influence the size distributions of the photochemically derived aerosols, depending on the concentration and size of the particulates. Gas to aerosol conversion rates and rates of change in the surface and volume distributions of the photochemical aerosols are compared for atmospheric and smog chamber data.