Physiologically structured population models are used to study the effects of nonpolar narcotic chemicals on two predator-prey community models. The biological focus is on the individual, which allows a discussion of exposure and effects, two necessary attributes of ecological risk assessment. Exposure is represented here by an exchange model between the environment and the organism with two possible routes — food and environment. Quantitative structure activity relationships (QSARs) are used to assess both lethality and chronic effects such as reduction in growth. Densities of lipid, carbohydrate and structure, variables used to describe an individual, are compiled to describe population dynamics. Variation among individuals is entirely responsible for the response of the population to toxic stress. Direct and indirect effects of stress on the community level of organization are observed for two model predator-prey systems: rainbow trout (Oncorhynchus mykiss) — Daphnia and Tetrahymena pyriformis — Escherichia coli. The direct effects include changes in the lipid, structure, age and size distributions of individuals in the populations. An example of an indirect effect is starvation of predators caused by death of prey in particular size classes.