Technical Abstract: Rangeland plants use carbon dioxide (CO2) in the process of photosynthesis to create high-energy organic compounds that are then used for their own growth, or are stored in plant tissues. These stored compounds such as sugar, starch, and cellulose are used as food by herbivores. Measurement of the net uptake of carbon dioxide by plants can provide a new way to measure ecosystem productivity on daily time scales. It also allows us to determine whether rangeland has the potential to capture and store some of the CO2 that is being released by man¿s use of fossil fuels. We used a technique called the Bowen-ratio energy balance method to measure daily CO2 uptake by plants on an ungrazed Wyoming big sagebrush community. Annual patterns of carbon dioxide uptake were very similar, with maximum rates occurring during May and June when soil moisture was high and soils had warmed. We found significant variations from year to year, with maximum rates in 1997 almost twice those observed in 1995 and 1996.