This prospective, randomized, double blind, placebo controlled study included fifty one patients (ASA I-III) scheduled for elective supratentorial mass resection surgery. The patients were randomized into three groups for investigation of the effects of mannitol alone and in combination with furosemide in different doses. Blood sodium, potassium, chlorine, lactate, urine and osmolarity levels were recorded. The brain relaxation score (BRS) was evaluated twice by the surgeon using a 4 point scale (1=very good, 2=good, 3=bad, 4=very bad); at dura opening, and 30 minutes after the administration of the study drug.