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 the 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 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. 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. The model assumes for all levels that a common threshold concentration exists at which the organism dies. 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.