Technical Abstract: The role of rangelands in regulating atmospheric CO2 concentrations is a critical issue in global climate change research. Rangelands are complex systems occupying more than 40% of the land area in the world and USA. To better understand this phenomena, we studied effects of burning and grazing on CO2 flux on a silty range site in a mixed-grass prairie with an Eapa fine loam soil. Treatments were: unburned-ungrazed control, burned in autumn 2003, and moderately grazed in May and June 2004 (dry spring) and 2005 (wet spring). Standing crop; leaf area (L); CO2 flux, at 1000 hours above 0.25 m2 of rangeland in an open chamber; photosynthetic active solar radiation (P); soil water (S); and soil and leaf temperatures, were sampled at about 30-day intervals from mid-April to mid-October. Green standing crop on control plots (April-June) ranged from 80 to 369 kg'ha-1 (2004) and 168 to 1172 kg'ha-1 (2005). Grazing removed an average of 35 to 43% green biomass and 45 to 50% leaf area. Day-time CO2 flux ('moles'm=2'sec-1) was best predicted by the relationship: CO2 flux = -5.2389+15.1689+1.754 E-2L+1.955 E-3 P-5.01 E-6 S*L-5.2389 L*P. All variables (P' 0.03) affected CO2 flux. All variables were affected (P'0.01) by the month within year interaction, but S was the only variable affected by treatment (P'0.05), with S significantly lower in burned than in grazed or control plots. Thus, weather, particularly precipitation, appears to exert primary control on CO2 exchange in the Northern Great Plains.