This research effort involved numerous ARS rangeland and pasture scientists from ten western states (ND, OK, TX, AZ, CO, WY, MT, UT, ID, OR) in cooperation with the Texas A&M University Blackland Research Center. The project focused on assessment of CO₂ flux over native rangeland at each of the locations. The Bowen ratio/energy balance (BREB) method was used to measure CO₂ flux, and supporting data included annual precipitation, peak leaf area index (LAI), standing biomass, litter, root biomass, and soil carbon. There were four years of data for most sites (generally 1996 to 1999) and all sites had at least three years of CO₂ flux data. Where estimates of net annual CO₂ fluxes have been completed it appears that rangelands are a sink for atmospheric CO₂. The average annual sequestration ranges from slightly less than 200 g CO₂ m^-2 yr^-1 at an arid sagebrush site to about 1100 g CO₂ m^-2 yr^-1 at a mesic tallgrass prairie site. There is large variation in yearly estimates, with most sites functioning as CO₂ sources during dry years and sinks during normal or wet years. Peak LAI and average annual precipitation accounted for 89% of the variation in average maximum monthly CO₂ flux across the measurement sites. Our results suggest that U.S. rangelands are likely a sink for atmospheric CO₂, although yearly weather patterns control sink/source relations within a site. Relatively easily obtained measurements (precipitation and LAI) hold promise for estimating CO₂ fluxes across rangeland ecosystems.