During dives with compressed air divers are exposed to environmental stress that leads to impaired endothelial function. Vascular changes linked to hyperoxia induced oxidative stress could be prevented by antioxidants. We wanted to complete a comprehensive observation on the endothelial function and isolate the impact of hyperoxia from other diving related stress factors in light of vitamin C supplementation. Fourteen male divers (age 37.6±8.5 years) with 15±10 years of diving experience participated in two studies. In the field study they completed a dive at 18 m seawater (msw), 49 minutes bottom time. At the laboratory they were exposed to ambient pressure gas mixture of 60% O2 balanced with pure N2 (same pO2 that occurs while breathing compressed air at 18 msw). Prior to both studies, half of the participants ingested ascorbic acid (1g) twice a week for six days and two hours before diving or O2 exposure, while the other half ingested placebo powder (1g). After two weeks of study interruption subjects switched groups and ingested the opposite type of powder before the studies. Endothelial function was assessed by measuring NO dependent flow-mediated dilation (FMD) pre -and post-intervention. All divers experienced a significant decrease in FMD% following placebo trials (from 9.18±1.35 to 5.97±1.36 post-dive, from 9.50±1.54 to 6.80±1.84 post hyperoxic breathing; p<0.001). FMD% did not change after 60% O2 breathing in the vitamin C trial (from 9.76±1.80 to 8.98±1.66; p>0.05), but was significantly decreased post-dive (from 9.88±1.53 to 7.90±1.64; p<0.001). There was no difference between SCUBA and hyperoxic conditions and no interaction effect. The data suggest that vitamin C supplementation can prevent hyperoxia induced decrease in endothelial function. Diving induced vascular dysfunction can only in part be explained by hyperoxia, as an important stress factor in diving.