Although mannitol is used for brain relaxation during neurosurgery and in the treatment of raised intracranial pressure; there is not a consensus on its safe and effective dose, the duration of its administration and its use in combination with loop diuretics. This study aimed to compare the effects of the mannitol alone and in combination with furosemide in different doses, on the brain relaxation, electrolyte, lactate levels of the blood, peroperative fluid balance and the volume of the urine in supratentorial mass resection surgeries.