This workshop was held at Denver, Colorado, May 20-24, 1979, to assist the Office of Pesticides and Toxic Substances in developing guidelines for subchronic toxicity testing under the Toxic Substances Control Act. The participants were organized into committees to discuss the relationship of protocol design to chemical class, the experimental design (route of exposure, dose, duration, test species, and age and number of animals), the evaluation of toxic effects (clinical observations, clinical laboratory tests, and pathology), the criteria for data extrapolation from one route of administration to another route, and limitations of acute and subchronic tests. Research recommendations submitted by each committee for their topic areas are presented in the document.