Nitrate reductase (NR; EC 1.6.6.1) activity increased at the beginning of the photoperiod in mature green maize (Zea mays L.) leaves as a result of increased enzyme protein level and protein dephosphorylation. In vitro experiments suggested that phosphorylation of maize leaf NR affected sensitivity to Mg2+ inhibition, as shown previously in spinach. When excised leaves were fed 32P-labeled inorganic phosphate, NR was phosphorylated on seryl residues in both the light and dark. Tryptic peptide mapping of NR labeled in vivo indicated three major 32P-phosphopeptide fragments, and labeling of all three was reduced when leaves were illuminated. Maize leaf NR mRNA levels that were low at the end of the dark period peaked within 2 h in the light and decreased thereafter, and NR activity generally remained high. It appears that light signals, rather than an endogenous rhythm, account primarily for diurnal variations in NR mRNA levels. Overall, regulation of NR activity in mature maize leaves in response to light signals appears to involve control of gene expression, enzyme protein synthesis, and reversible protein phosphorylation.