Frequencies of detection, concentrations and annual loadings of 50 selected toxic chemicals have been studied in urban runoff from the Canadian Great Lakes Basin. The mean concentrations of these chemicals in stormwater and street sediment were determined in a field program conducted in 12 urban centres in the study area. Using these mean concentrations and the computed runoff volumes and sediment yields, annual loadings of toxic chemicals in urban runoff from the study area were estimated within an order of magnitude. The highest basin loadings were computed for zinc (300 t/yr), lead (100 t/yr), and some other inorganics. Among organic substances, the highest individual loadings were computed for several poly-nuclear aromatic hydrocarbons (e.g., fluoranthene - 200 kg/yr), followed by PCBs (80 kg/yr), some chlorinated benzenes (1,2-dichlorobenzene - 30 kg/yr), and organochlorine pesticides (alpha-BHC - 12 kg/yr). Such loadings do not include possible contributions from combined sewer overflows or illicit point source discharges into storm sewers.