Laboratory ecosystems have been used to test a model for the fate of toxic chemicals. Two questions are asked in such a test. First, are the laboratory systems functional ecosystems in which primary productivity, secondary productivity, decomposition, element cycling, and various synergisms occur. Second, is the model capable of predicting the fate of the chemical in the laboratory ecosystem. The answer to the first question is important in the interpretation and potential extrapolation of the answer to the second. These relationships have been considered and evaluated with the conclusion that laboratory systems can be ecosystems provided certain operational requirements are met. The model (the basis of the Exposure Analysis Modeling System, EXAMS) is not rejected using ordinary statistical analysis, and, based on arguments founded upon fundamental concepts in physical chemistry, it is accepted as a working theory of the behavior of chemicals in the environment.