Aim:This study aims to investigate changes in nitric oxide (NO) levels, oxidative stress, and dynamic thiol/disulphide homeostasis in the serum and cerebrospinal fluid (CSF) of patients with aneurysmal subarachnoid hemorrhages (aSAH). Material and Methods:This prospective study included a total of 40 consecutive patients suffering from aSAH, who were operated on within the first 48 hours from onset of symptoms; CSF and blood samples were collected from these patients during their operations. To create a control group, blood samples and cerebrospinal fluid were taken from patients (n=40) without neurologic disorders who had undergone lumbar puncture for spinal anesthesia.Results:We observed that the serum total antioxidant status had decreased markedly (P=0.0143) but that no change was evident in the oxidative stress index and total oxidant status in aSAH patients when compared to the controls. While total thiol (P=0.0014) and native thiol (P<0.0001) levels had decreased in the aSAH patients, disulphide levels (P<0.0001) had increased significantly. Although the native thiol/total thiol ratio declined (P<0.0001), the dynamic disulphide/total thiol ratio (P<0.0001) and dynamic disulphide/native thiol ratio (P<0.0001) increased markedly in serum samples from the patient group. Patient serum NO levels were also significantly elevated (P<0.0001). There were no marked changes in CSF for all measured parameters (P>0.05). Conclusion:Our data demonstrated that serum NO levels and oxidative stress parameters increased markedly in the patients. These results may help to understand the underlying mechanisms behind early tissue damage due to aSAH and to monitor disease progression and improve the early detection of disease severity.