Eastern Oregon sagebrush steppe vegetation is characterized by an understory of herbaceous grasses and forbs with an overstory that includes several shrub species. The most common species is Wyoming big sagebrush. This community type is one of the largest contiguous plant assemblages in the continental United States. Little is known regarding the dynamics of CO₂ flux over these rangelands. We established a study at the Northern Great Basin Experimental Range in 1995 to determine the magnitude of CO₂ flux on an ungrazed sagebrush-steppe that is representative of large areas of the northern Great Basin. Bowen ratio/energy balance was used to obtain 20-minute summaries of energy, H₂O, and CO₂ fluxes. We will discuss CO₂ fluxes in this presentation and will use the ecophysiological convention in which positive fluxes are toward the surface. Maximum CO₂ assimilation rate measured on our site was about 0.5 mg CO₂ m^-2 s^-1, usually between 15 May and 1 June. Maximum respiration rate also occurred within the same time period and averaged about –0.4 mg CO₂ m^-2 s^-1. Daily uptake peaked in late May most years at about 9 g m^-2 d^-1. Our site was a moderate sink during most seasons, however in 1996 we measured a net loss of CO₂ from the surface, apparently a result of freezing nighttime temperatures in mid June. Below-average precipitation was received in 2000, and during that year the site again became a source for CO₂. The dataset illustrates the high variability of CO₂ uptake associated with northern Great Basin ecosystems. This variability most likely results from the extreme inter-annual variation of precipitation and temperature that occurs in this ecosystem. These data suggest that this site is a sink for CO₂ during average to wet years, but becomes a source during drought or after a disturbance such as a late frost.