Objective: The aim of this study is to investigate the effect of routine administration of intravenous cristalloid and colloid fluids on subarachnoid block induced hypotension and cerebral oxygenization. Material and Methods: Seventyfive, 18-50 years old, ASA I patients that were scheduled for operation under spinal anesthesia were enrolled into the study. Patients were divided into 3 groups after standard monitoring. G1; intravenous prehydration with 4%Gelatine Polysuccinate before spinal anesthesia, G2; intravenous prehydration with Ringers Lactate s before spinal anesthesia, G3; no prehydration group. After spinal anesthesia, fluid maintenance was provided with intravenous Ringer Lactate at a dose of 5 ml/kg/h in all patients. Systolic blood pressure levels under 90 mmHg or 20%decrease from the baseline was accepted as hypotension. Complications such as changes in haemodynamic parameters, nausea, vomiting and shivering were recorded with an interval of 5 minutes during the operation. Bilateral Near Infrared Spectroscopy monitoring was performed for cerebral oxygenation. Results: It was observed that intravenous prehydration significantly decreased the nausea and vomiting before spinal anesthesia (p<0.05), whereas in G1 shivering was significantly less (p<0.05). While ephedrine was used in 15 patients (60%) in G3, it was observed that intravenous prehydration decreased ephedrine need significantly (p<0.05). As the operation time increased, cerebral oxygenation was significantly decreased in all three groups and there was no difference between groups in terms of right and left lobe brain oxygenation. There was a significant decrease in SAP, DAP and MAP values in all three groups as the operation time increased, but there was no significant difference between the groups. Conclusion: Although there is no difference between the use of colloid or crystalloid solutions in terms of persistence of cerebral oxygenation under spinal anesthesia, the use of ephedrine in the non-hydrated group provides similar hemodynamic stability and the brain oxygenation is maintained.