The effect of biotic stress induced by viral infection ( Potato virus Y, strain NTN and O) on NADP-malic enzyme (EC 1.1.1.40) in tobacco plants ( Nicotiana tabacum L., cv. Petit Havana, SR1) was tested at the transcriptional, translational and activity level. The increase of enzyme activity in infected leaves was correlated with the increased amount of expressed protein and with mRNA of cytosolic NADP-ME isoform. Transcription of the chloroplastic enzyme was not influenced by viral infection. The increase of the enzyme activity was also detected in stems and roots of infected plants. The effect of viral infection induced by Potato virus Y, NTN strain, causing more severe symptoms, was compared with the effect induced by milder strain PVYO. The observed increase in NADP-malic enzyme activity in all parts of the studied plants was higher in the case of PVYNTN strain than in the case of strain PVYO. The relevance of NADP-malic enzyme in plants under stress conditions was discussed.