Inducible nitric oxide synthase (iNOS) gene expresses a calcium calmudolin-independent enzyme which can catalyse NO production from L-arginine. The induction of iNOS activity has been demonstrated in a wide variety of cell types under stimulation with cytokines and lipopoly saccharide (LPS). Previous studies indicated that all nitric oxide synthases (NOS) activated in human umbilical vein endothelial cells (HUVEC) were calcium dependent. In this work we indicate that endothelial cells can express iNOS mRNA under stimulation by cytokines, LPS and H2O2. We also found that iNOS mRNA expression has increased by augmentation of H2O2 concentration and its expression depends on increasing in time. Therefore we conclude that two isoforms of NOS are expressed in the HUVEC and induction of iNOS enzyme by H2O2 and NO production in endothelial cells may represent a novel cellular defence mechanism against oxidative stress and infections.