An ecological impact model, designed to estimate the impact of chemically treated and untreated oil spills on seabirds and fish, was developed to aid dispersant-use decision-making. Impact was defined as the proportion of the animal population killed and the length of time required for that population to recover to its prespill condition. Subroutines dealt with the fate of oil, the toxicity of spilled oil, the concentration of organisms at the location of the spill, and the recovery potential of the target population. The model was site-specific. The paper describes briefly the structure and operation of the model, but most of the discussion deals with the modeling of eco-toxicity. The model uses information on oil fate and the sensitivity of organisms to compute the size and location of lethal areas. For fish, the dose of hydrocarbons reaching the organism's tissues is estimated to determine whether this dose is lethal or not. For seabirds, it was assumed that any contact between seabirds and the oil slick would be lethal. Our findings suggest that for some situations, chemically dispersed oil might have greater area of effect and greater environmental impact than undispersed oil. More important, the area of effect of large oil spills, whether treated or not, may be very small relative to the area over which target populations are distributed. Estimates of the size of the zone of lethal conditions made using the exposure dose-response method developed here were similar to those obtained using other methods such as the critical-peak-exposure concentration method and the integrated exposure (Toxicity Index) method.