Introduction: Vasodilation in response to hypoxia is mainly mediated by cyclooxygenase (COX) activation and production of prostacyclin (PGI2) (1). However, there are evidences that cytochrome P450-epoxygenase metabolites contribute to vasodilation in healthy vessels (2). Materials and Methods: Hyperbaric oxygenation (HBO2) was performed in a chamber with 100% O2 (2 bar) 2 hours/day for 4 consecutive days. Male Sprague-Dawley rats (N= 5) were divided in 4 groups; control, diabetic (DM) (DM induced by streptozocin 60mg/kg i.p.), HBO2 treated DM rats (DM+HBO2) and HBO2-control rats. On the 5th day, prior to decapitation, rats were anesthetized with ketamin-chlorid (75 mg/kg) and midazolam (0.5 mg/kg). Isolated middle cerebral arteries (MCA) were mounted on glass pipettes of DMT 110P pressure myograph system for internal diameter measurements in normoxia (21% O2 in superfusate and perfusate), and reduced pO2 (0%) conditions. To assess the role of CYP450-epoxygenases’ vasodilating metabolites (EETs), the CYP450 inhibitor -MS-PPOH (10-5M) was used. To assess the role of COX metabolites, indomethacin (INDO) (10-5M) was used. The data were analyzed with One-way ANOVA and presented as mean±SEM (microns); p<0.05 was considered statistically significant. The Ethical Committee of Faculty of Medicine University of Osijek approved the study. Results: MCA of control rats exhibit significant vasodilation (23±3) compared to DM rats which failed to dilate in response to reduced pO2 (0±3). INDO or MS-PPOH did not affect this lack of response in DM rats. Vasodilation was restored in DM+HBO2 rats (14±2). In the presence of MS-PPOH, vasodilation to reduced pO2 was diminished in control group (11±1) and more inhibited in control + HBOT group (4±1), while vasodilation was eliminated in DM+HBO2 group (-6±2) and in DM group (-6±2). INDO eliminated dilation to reduced pO2 in control (-5±4) and DM (-2±6) groups but only partly reduced it in both HBO2 groups. Conclusion: Partially preserved vasodilation in the presence of INDO in HBO2 groups, and eliminated vasodilation in response to reduced pO2 in the presence of MS-PPOH in both HBO2 groups suggest that HBO2 affects CYP450-epoxigenase in MCAs of healthy and DM rats and shifts vasodilatory mechanisms in response to reduced pO2 towards EETs mediated pathways. Support: This study was supported by grant of Ministry of Science, Education and Sports, Croatia, #219-2160133-2034.