Individually caged asellids placed in urban receiving waters were subjected to discharges from stormwater and combined sewer overflow outfalls. In each field trial the volume of precipitation and the number of dry days were recorded on a weekly basis and the metal (copper, zinc, cadmium, lead) body concentrations of caged organisms were measured. Using Principal Component Analysis (PCA), combinations of the measured parameters which explained much of the variation in organism mortality or changes in the weight of caged asellids were isolated. Three different sets of variable combinations were identified, two of which explained significant independent sources of variation responsible for changes in organism weight and one which explained a significant source of the variation in asellid mortality. Organism mortality and weight change were selected as dependent variables and regressed against the principal components of the independent variable predictors to produce empirical first order regression equations. Changes in the weight of caged organisms were found to result from the joint interaction of a number of identified variables. Copper was identified as being the most important metal toxicant with respect to mortality. Meteorological variables were found to have a greater influence on organism mortality than upon changes in organism weight.