Elevated CO2 (ambient + 35 Pa) increased shoot dry mass production in Avena fatua by apprx 68% at maturity. This increase in shoot biomass was paralleled by an 81% increase in average net CO-2 uptake (A) per unit of leaf area and a 65% increase in average A at the 'ecosystem' level per unit of ground area. Elevated CO2 also increased 'ecosystem' A per unit of biomass. However, the products of total leaf area and light-saturated leaf A divided by the ground surface area over time appeared to lie on a single response curve for both CO2 treatments. The approximate slope of the response suggests that the integrated light saturated capacity for leaf photosynthesis is apprx 10-fold greater than the 'ecosystem' rate. 'Ecosystem' respiration (night) per unit of ground area, which includes soil and plant respiration, ranged from -20 (at day 19) to -18 (at day 40) mu-mol m-2 s-1 for both elevated and ambient CO2 Avena. 'Ecosystem' below-ground respiration at the time of seedling emergence was apprx -10 mu-mol m-2 s-1, while that occurring after shoot removal at the termination of the experiment ranged from -5 to -6 mu-mol m-2 s-1. Hence, no significant differences between elevated and ambient CO2 treatments were found in any respiration measure on a ground area basis, though 'ecosystem' respiration on a shoot biomass basis was clearly reduced by elevated CO2. Significant differences existed between leaf and 'ecosystem' water flux. In general, leaf transpiration (E) decreased over the course of the experiment, possibly in response to leaf aging, while 'ecosystem' rates of evapotranspiration (ET) remained constant, probably because falling leaf rates were offset by an increasing total leaf biomass. Transpiration was lower in plants grown at elevated CO2, though variation was high because of variability in leaf age and ambient light conditions and differences were not significant. In contrast, 'ecosystem' evapotranspiration (ET) was significantly decreased by elevated CO2 on 5 out of 8 measurement dates. Photosynthetic water use efficiencies (A/E at the leaf level, A/ET at the 'ecosystem' level) were increased by elevated CO02. Increases were due to both increased A at leaf and 'ecosystem' level and decreased leaf E and 'ecosystem' ET.