Aim: The aim was to investigate the interactions of hyperbaric oxygenation (HBO) and sulphur dioxide (SO2) inhalation (at 10 ppm, one hour daily for 45 days) on lipoperoxidation and fragility of RBC membranes. Methods: Malonyldialdehyde (MDA) levels, osmotic fragility ratios and hematologic parameters of a total of 24 rats were compared in four different groups controls, HBO, SO2 and (HBO+SO2). HBO was applied at 3 Atm abs (303.9 kPa), 1 hour daily, for 45 days in a specifically designed exposure chamber. Results: MDA levels and osmotic fragility ratios were significantly higher in groups-B, C and D than in control group (p<.05 for all). Marked decrease in MDA levels and osmotic fragility ratios were observed in group-D (HBO+SO2) compared to groups-B and C (p<0.05). However, MDA levels and osmotic fragility ratios in group-D were also significantly higher than in groupA (p<0.05 for all). Conclusion: HBO-treatment or SO2 inhalation alone resulted in structural and functional oxidant damage, indicated by higher MDA levels and osmotic fragility ratios, but when they were applied together, decreases in oxidant damage in RBC membranes were observed indicating HBO may also be useful in the cities with a serious air pollution problem, at least it does not cause additional oxidant stress.