Exposure of the leaf canopy of corn seedlings (Zea mays L.) to atmospheric CO(2) levels ranging from 100 to 800 mul/l decreased nitrate accumulation and nitrate reductase activity. Plants pretreated with CO(2) in the dark and maintained in an atmosphere containing 100 mul/l CO(2) accumulated 7-fold more nitrate and had 2-fold more nitrate reductase activity than plants exposed to 600 mul/l CO(2), after 5 hours of illumination. Induction of nitrate reductase activity in leaves of intact corn seedlings was related to nitrate content. Changes in soluble protein were related to in vitro nitrate reductase activity suggesting that in vitro nitrate reductase activity was a measure of in situ nitrate reduction. In longer experiments, levels of nitrate reductase and accumulation of reduced N supported the concept that less nitrate was being absorbed, translocated, and assimilated when CO(2) was high. Plants exposed to increasing CO(2) levels for 3 to 4 hours in the light had increased concentrations of malate and decreased concentrations of nitrate in the leaf tissue. Malate and nitrate concentrations in the leaf tissue of seven of eight corn genotypes grown under comparable and normal (300 mul/l CO(2)) environments, were negatively correlated. Exposure of roots to increasing concentrations of potassium carbonate with or without potassium sulfate caused a progressive increase in malate concentrations in the roots. When these roots were subsequently transferred to a nitrate medium, the accumulation of nitrate was inversely related to the initial malate concentrations. These data suggest that the concentration of malate in the tissue seem to be related to the accumulation of nitrate.