The potential for, and mechanism of, [CO2] effects upon C4 plants has received considerable research interest but remains poorly understood. In 2002 and 2004, a rainfed-field experiment utilizing FACE technology was undertaken, in the U.S. Corn Belt, to determine the effects of elevated [CO2] on Zea mays. FACE allows experimental treatments to be imposed upon a complete soil-plant-atmosphere continuum with none of the effects of experimental enclosures on plant microclimate. Each year, crop performance was compared at ambient [CO2] (~370 ppm) and the elevated [CO2] (~550 ppm) predicted for 2050. The diurnal course of gas exchange of upper canopy leaves was measured in situ across the growing season of 2002. This was repeated in 2004 along with analysis of carbon and nitrogen metabolism, water relations, growth and yield. First, this tested if elevated [CO2] would directly: (1) stimulate C4 photosynthesis, and (2) reduce stomatal conductance and, therefore, crop water use. Second, the experiments also tested if altered water relations under elevated [CO2] could feedback to enhance carbon gain during water stress. 2004 was unusual climatically and provided a unique opportunity to test the effects of elevated [CO2] in the absence of water stress. There was no [CO2] effect on photosynthesis, carbon metabolism, growth or yield. Nevertheless, elevated [CO2] reduced stomatal conductance, transpiration and soil moisture depletion. 2002 was an “average” year in which plants experienced episodic water stress. During these dry periods, photosynthesis was greater under elevated [CO2]. We conclude, elevated CO2 can only indirectly enhance carbon gain during drought. In 2006 the experiment will address the mechanism by which elevated [CO2] ameliorates drought stress on photosynthesis.