Total pyridine nucleotide concentration of root tissue for young soybean (Glycine max var. Bansei) and sunflower (Helianthus annuus L. var. Mammoth Russian) plants is the same with either ammonium or nitrate, but nitrate results in an increased proportion of total oxidized plus reduced NADP (NADP[H]) seemingly at the expense of NAD. The activity of NADH- and NADPH-dependent forms of glutamic acid dehydrogenase is correlated with the ratio of total oxidized plus reduced NAD to NADP(H). The low NAD: NADH ratio maintained in nitrate roots despite active NADH utilization via nitrate reductase and glutamic acid dehydrogenase may be the result of nitrate-stimulated glycolysis. Nitrate roots also maintain a high level of NADPH, presumably by the stimulatory effect of nitrate utilization on glucose-6-phosphate dehydrogenase activity. In the presence of nitrate rather than ammonium, the highly active nitrate-reducing leaves of soybean show a greater proportion of total pyridine nucleotide in the form of NADP(H) than do the inactive leaves of sunflower.For all tissues examined, ammonium nutrition yields a higher concentration of total adenine nucleotide than is found with nitrate. The data indicate the production of a higher level of metabolites that enter into purine synthesis with ammonium than with nitrate. Glutamine synthetase activity can be correlated with the concept that enzymes utilizing ATP for biosynthetic purposes increase in activity in accordance with the energy level of the cell.