The validity and practical implications are discussed of the proposition that CO2 enrichment of the leaf environment enhances plant growth and, simultaneously, decreases plant water use. A theoretical analysis of the water and carbon dioxide balance of plant leaves was made in the form of a computer program based upon known physiological facts. It predicts significant increases in water use efficiency by plants as CO2 is enriched, the size of the increase depending upon the external conditions. Experimental tests were conducted in an environmental simulator with stands of soybean, pepper and southern pea plants. The predictions of the model were substantially verified, with CO2 concentrations ranging from normal to six-fold normal. Although CO2 is obviously an ideal antitranspirant, the efficacy of its release in open stands is doubtful in view of plausible economic factors. But, in enclosures this would be a different matter, and for such situations a scientific basis is given for engineering and systems analysis.