Technical Abstract: We examined plant growth, photosynthesis and leaf constituents of both the wild type (WT) and two mutant lines of barley (Hordeum vulgare L. cv. Steptoe) with defects in NADH-dependent nitrate reductase (NADH-NAR) activity. The first mutant, nar1, had a lesion within the NAR structural gene and the second, nar2, had a defect in the molybdopterin subunit, which rendered it pleiotropic. Total biomass was reduced 17 and 43% for nar1 and nar2, respectively, in comparison to the WT when measured 8 weeks after sowing in a greenhouse experiment. Foliar soluble protein also was 22 to 23% lower in the two mutants compared to the WT. Whole plant growth results were similar whether or not nitrate was the sole source of inorganic nitrogen. All three genotypes displayed accelerated plant growth rates in response to continuous CO2 enrichment. Trace levels of NADH-NAR and NAD(P)H-NAR activity, the bispecific form of the enzyme, were present in the two mutant lines. Net rates of CO2 assimilation measured at both growth CO2 partial pressures were only slightly decreased by a defect in the NADH-NAR structural gene alone. Levels of soluble sugars and starch were similar in leaves of all three genotypes grown at 38 Pa CO2. The above results indicated that a defect in NADH-NAR primarily affected nitrogenous leaf constituents, whereas effects on growth, carbohydrate metabolism and photosynthesis were small. Differences between nar1 and nar2 were likely the result of pleiotropism. The presence of bispecific NAD(P)H-NAR in the two mutants likely compensated for the lack of NADH-NAR. Although this enzyme was present in trace amounts, its activity was sufficient to allow nearly normal growth in both the ambient and elevated CO2 treatments.