Two isoforms of NADP-malic enzyme have been characterized in maize leaves. The 72 kDa-form of the protein, present mainly in etiolated maize leaves, has lower specific activity than the 62 kDa-form, which is implicated in C4 metabolism and predominates in green leaves. The larger form of the enzyme has higher Km values for NADP and malate and lower PH optimum. The antibodies raised against the 62 kDa-form of the protein react with the 72 kDa-form. Steady state levels of NADP-malic enzyme, as measured by the amount of protein and activity, increase several-fold when dark-grown maize seedlings are illuminated. This increase in protein is about 13-fold for the 62 kDa-form of the enzyme, while the 72 kDa-form remains practically constant after a transient increase. Northern blot analysis using a specific probe against the 62 kDa-form of the enzyme, reveals the increase of a 2.2 kb mRNA during greening. Southern hybridization analysis with genomic DNA suggests the presence of more than one gene encoding NADP-malic enzyme in maize. In this paper we provide biochemical and inmunological evidence suggesting that both isoforms are closely related and that the 72 kDa-form is also present in low levels in mature green leaves.