The paper addresses the compilation of a 1980 emissions inventory for use in the National Acid Precipitation Assessment Program (NAPAP). The current inventory (Version 4.0) contains point source data for 50,200 plants with more than 201,000 emission points and area source data for the 3,069 counties in the 48 contiguous States and the District of Columbia. Emissions of SO2, NOx, VOC, sulfates, ammonia, CO, and particulates are included in the inventory, but the paper focuses on SO2, NOx, and VOC which are of primary interest for acid deposition research. NAPAP Version 4.0 emissions of SO2, NOx, and VOC are 27.1, 23.7, and 23.3 million tons per year, respectively. Emissions in the NAPAP data base are in reasonable agreement with the U.S./Canada Work Group 3B and the EPA Office of Air Quality and Standards emissions trends estimates. NAPAP fuel use data show reasonable agreement with fuel values in DOE's State Energy Data Report. Version 4.0 of NAPAP represents the best detailed inventory of emissions on a national scale that has been developed to date. Nevertheless, additional improvements are planned, focusing on major point sources.