The appearance and disappearance of NADH:nitrate reductase (NR) in the leaves of corn (Zea mays L. W64A x W182E) were studied using activity assays, an enzyme-linked immunosorbent assay (ELISA) and western blotting. N-starved, etiolated corn plants were treated with nutrients containing either 35 millimolar NH(4)-nitrate or K-nitrate and immediately thereafter given light. The curve for enhancement of NR activity had three phases: 1 hour lag, 5 hour rapid increase, and steady state. The pattern for NR protein, as measured with the ELISA, also had three phases, but the increase was more rapid and the steady state was established earlier. To differentiate the effects of N nutrition from those of light, N-starved etiolated plants were given N nutrients 4 hours before light. During the dark pretreatment, NR activity and protein increased. When the light was turned on the NR activity and protein increased very rapidly without a lag. Western blots of polyacrylamide gels of native and denatured crude extracts showed that NADH:NR polypeptide was absent prior to treatment with N nutrients, but appeared after nitrate was given in dark or light. A low level of NR activity was found in N-starved, etiolated plants and it was shown by western blotting to be an NR form with a different electrophoretic mobility in nondenaturing gels. Since this minor NR form was not influenced by either nitrate or light, it was designated a constitutive NR. Dark decay of NR activity and protein was also studied. After the plants which had been in light with N nutrients for 24 hours were transferred to dark, the NR activity dropped by 30% within 1 hour, but the NR protein did not decrease. This inactivation of NR was further supported by returning the plants to the light after 1.5 hours of dark and finding the activity restored without change in NR protein. After the initial activity drop, a parallel decrease in NR activity and protein was observed, which was likely due to irreversible degradation by proteolysis.