Three dicarbonyl reagents were used to demonstrate the presence of an essential arginine residue in the NO3- uptake system from corn seedling roots (Zea mays L., Golden Cross Bantam). Incubation of corn seedlings with 2,3-butanedione (0.125-1.0 millimolar) and 1,2-cyclohexanedione (0.5-4.0 millimolar) in the presence of borate or with phenylglyoxal (0.25-2.0 millimolar) at pH 7.0 and 30°C resulted in a time-dependent loss of NO3- uptake following pseudo-first-order kinetics. Second-order rate constants obtained from slopes of linear plots of pseudo-first-order rate constants versus reagent concentrations were 1.67 × 10-2, 0.68 × 10-2, and 1.00 × 10-2 millimolar per minute for 2,3-butanedione, 1,2-cyclohexanedione, and phenylglyoxal, respectively, indicating the faster rate of inactivation with 2,3-butanedione at equimolar concentration. Double log plots of pseudo-first-order rate constants versus reagent concentrations yielded slope values of 1.031 (2,3-butanedione), 1.004 (1,2-cyclohexanedione), and 1.067 (phenylglyoxal), respectively, suggesting the modification of a single arginine residue. The effectiveness of the dicarbonyl reagents appeared to increase with increasing medium pH from 5.5 to 8.0. Unaltered Km and decreased Vmax in the presence of reagents indicate the inactivation of the modified carriers with unaltered properties. The results thus obtained indicate that the NO3- transport system possesses at least one essential arginine residue.