Erythrocyte deformability is one of the most important charactheristics of erythrocytes for an effective microcirculatory function and is affected from a number of factors, including the oxidative-damage-induced by nitric oxide (NO). This study was performed to investigate the effects of in vitro melatonin incubation on the antioxidant status and deformability of erythrocytes in sodium nitroprusside (SNP), a nitric oxide donor, induced oxidative stress. 40 blood samples taken from the adult healthy people were divided into 4 groups randomly and incubated with saline, SNP (1 mM), melatonin (MEL, 1 mM), MEL+SNP and SNP+L-NAME (5 mM) respectively. Relative filtration rate (RFR), relative filtration time (RFT) and relative resistance (Rrel) were determined as the indexes of erythrocyte filterability. In addition, malondialdehyde (MDA, as an index of lipid peroxidation) and the antioxidant activities of glutathione peroxidase (GSH-Px), superoxide dismutase (SOD) and catalase (CAT) were also determined in the red blood cells of all groups revealing the oxidant-antioxidant activity. RFT and the Rrel of the erythrocytes incubated with SNP increased significantly (p<0.05) whereas the RFR of the erythrocytes decreased (p<0.05) in comparison to all groups. This reduction in RFR was prevented with both L-NAME or MEL incubation. Furthermore, MEL was found to be significantly efficient in preventing the erythrocytes from lipid peroxidation in these groups. In addition, GSH-Px and SOD activities were elevated with SNP incubation reflecting the oxidative stress in erythrocytes, whereas the CAT activity remained unchanged. Melatonin has no significant effect on the GSH-Px and CAT activity but, it caused a significant decrease in SOD activity (p<0.05). These results reveal that, melatonin can protect the erythrocytes from impaired deformability in SNP-induced oxidative stress due to antioxidant effects as revealed by lipid peroxidation and antioxidant enzyme activities.