The paper describes the methodology that was used to develop the 1985 National Acid Precipitation Assessment Program (NAPAP) Emissions Inventory, including quality control procedures, and summarizes the inventory contents. Development of the 1985 inventory required detailed investigation of millions of sources of air pollution. Data gathered for the inventory include activity levels (e.g., fuel consumed in a boiler, tons of product produced at a manufacturing plant, and miles traveled for transportation sources), emission estimates, and, for point sources, physical parameters (such as stack height) that affect the atmospheric transport of emissions. The inventory was produced in two formats, an annual data base and a modelers' inventory. The annual data base fulfills the requirements of the NAPAP assessment activities and provides a baseline inventory for the emission projection models, while the modelers' inventory was developed for use as input to atmospheric models. The 1985 inventory provided a considerable amount of data useful for characterizing emissions sources. The inventory indicated that, in 1985, over 75% of U.S. SO2 emissions originated from large plants (mostly electric utility plants) that emitted at least 10 Gc(10,000 tons) annually. In comparison, only 27% of national NOx and 1% of national VOC emissions came from such large plants.