The capability to predict the environmental fate of chemicals based on their chemical properties is well developed and widely practiced. Poorly developed, however, is the capability to predict the effects of those chemicals. To address this deficiency, significant effort has been directed to the creation of mathematical models to predict the effects of toxicants in aquatic systems. These models consist of two parts: an ecological component and a toxicological component. The ecological components are organized at levels of integration and resolution intended to be relevant to the kinds of expected problems and questions to be addressed in determining whether to permit or prohibit the use of a new chemical. Models of systems in which the organisms are assumed to have reached steady state with respect to the level of toxicant are at the coarsest level of resolution. Populations are represented at an intermediate and ecosystems at a fine scale of resolution. A toxicological model in corresponding detail is associated with each level to provide predictions of effects. This model assumes for all levels that a common threshold concentration exists at which the organism dies. With this assumption, a model for exchange with the environment and assumptions of rapid internal distribution of the toxicant, the time to death for an individual depends on the fraction of its body comprised of fat. Probability of death by the end of a fixed exposure time is a function of the statistical distribution of the fraction of fat in the organisms' bodies.