Plant bioassays are used to predict the effects of chemicals on terrestrial systems. However, in tiered-testing schemes, plants are tested only when introduction and fate information suggest that terrestrial exposure is probable. In this paper we discuss how information on chemical introduction and fate may trigger the need for plant toxicity tests, and how terrestrial exposure estimates are obtained and used at the U.S. Food and Drug Administration (FDA). FDA regulates chemicals used for food additives, food packaging and processing, and animal drugs and feed additives. These materials may be introduced into the environment as a result of their manufacture, use, or disposal. Information on introduction rates, environmental partitioning, and transformation is used to estimate the potential for terrestrial exposure. Described in detail are two scenarios for terrestrial exposure from application of wastewater treatment plant sludges to soils and one scenario for direct introductions of animal wastes to soils.