Urban highway runoff samples collected from four rainfall drains were analyzed for heavy metals and polycyclic aromatic hydrocarbons (PAHs). The concentrations of constituents varied widely during the history of each storm and appeared to be highest in the first runoff water. Results showed that iron and aluminum were the principal constituents of particulates. Cadmium, chromium, copper, manganese, lead and zinc were mainly particulate-bound while nickel was mainly in dissolved form. The higher molecular weight PAHs were more associated with suspended solids in the runoff. Predominant PAHs-phenanthrene, fluoranthene and pyrene-comprised about 50% of fifteen quantified PAHs constituents in each sample. In the events where peak flush occurred during the initial phase, more than half the total load flowed out during the initial phase of runoff flow. In this case, initial runoff water treatment was believed to be effective. In results from Ames assay, mutagenicity was appreciably associated with PAHs in the particulate fraction of runoff water. The dissolved fraction also showed positive mutagenic response by unknown soluble aromatic compounds.