We previously showed that the selective accumulation of phosphoenolpyruvate carboxylase (PEPC) in photosynthetically maturing maize (Zea mays L.) leaf cells induced by nitrate supply to nitrogen-starved plants was primarily a consequence of the level of its mRNA (B Sugiharto, K Miyata, H Nakamoto, H Sasakawa, T Sugiyama [1990] Plant Physiol 92: 963-969). To determine the specificity of inorganic nitrogen sources for the regulation of PEPC gene expression, nitrate (16 millimolar) or ammonium (6 millimolar) was supplied to plants grown previously in low nitrate (0.8 millimolar), and changes in the level of PEPC and its mRNA were measured in the basal region of the youngest, fully developed leaves of plants during recovery from nitrogen stress. The exogenous supply of nitrogen selectively increased the levels of protein and mRNA for PEPC. This increase was more pronounced in plants supplemented with ammonium than with nitrate. The accumulation of PEPC during nitrogen recovery increased in parallel with the increase in the activity of glutamine synthetase and/or ferredoxin-dependent glutamate synthase. Among the major amino acids, glutamine was the most influenced during recovery, and its level increased in parallel with the steady-state level of PEPC mRNA for 7 hours after nitrogen supply. The administration of glutamine (12 millimolar) to nitrogen-starved plants increased the steady-state level of PEPC mRNA 7 hours after administration, whereas 12 millimolar glutamate decreased the level of PEPC mRNA. The results indicate that glutamine and/or its metabolite(s) can be a positive control on the nitrogen-dependent regulation of PEPC gene expression in maize leaf cells.