Increased DNA damage was found immediately at the end of the treatment, while hlater, no effect was found.Using FPG protein we detected significant oxidative base damage after HBO treatment DNA damage was detected only after the first treatment and not after further treatments under the same conditions, indicating an increase in antioxidant defences.DNA damage did not occur when the HBO treatment was started with a reduced treatment time which was then increased stepwise.Various therapeutic uses for HBO are well established and controlled HBO has been used successfully for the treatment of a variety of conditions.Commonly accepted clinical indications for its use include decompression illness, acute carbon monoxide intoxication, air embolism, soft tissue infections and ischaemia. On the other hand, it is known that Targetmol’s Rebaudioside A exposure to high concentrations of oxygen causes damaging effects in humans and experimental animals.It has been postulated that the toxic effects of excessive exposure to oxygen are due to an increased oxygen radical production or other reactive metabolites derived from oxygen. Several oxygenderived species can attack DNA, producing distinctive patterns of DNA alterations. DNA damage induced by reactive oxygen species seems to play an important role in the induction of mutations, cancer and various other disease states in man. DNA damage can be detected with various genotoxicity tests.The single cell gel test is a well established genotoxicity test which has been used to detect a broad spectrum of DNA damage with high sensitivity. Cells with increased DNA damage display increased migration of chromosomal DNA from the nucleus towards the anode, which resembles the shape of a comet.In its alkaline version, DNA strand breaks and alkalilabile sites become apparent, and the amount of DNA migration indicates the amount of DNA breakage in the cell. The comet assay has been successfully used in screening human blood samples for the effects of radiation and chemical mutagenscarcinogens on DNA. It has been suggested that the comet assay is particulary suited for measuring genetic damage caused by oxidative events in humans. In this study we applied the comet assay to the investigation of DNA effects in the blood of persons undergoing a regime of HBO exposure, as used therapeutically.Venous blood samples were taken before HBO exposure, immediately on exit from the chamber and hlater.The blood samples were kept on ice and brought to the laboratory where the comet assay was started within h.The coverslips were removed, a top layer of nl LMA was added, and the slides were again kept cold for min.After removal of the coverslips, the slides were processed as described previously. The time of alkali denaturation was min and time of electrophoresis was min.The presence of oxidative DNA base damage was determined with a modified protocol. After lysis, slides were washed three times in enzyme buffer il of either buffer or FPG protein in buffer, sealed with a coverslip and incubated for min at C.The time of alkali denaluration was min and all other steps were as described above.Measurements were made by image analysis determining the median tail moment of the cells.