 30 healthy women and 30 healthy men, who are scheduled for surgery of the middle ear or parotic gland are included in the study. Within each gender group, the subjects are randomized to receive perioperative intravenous fluid regimes either based on body weight or lean body mass LBM. The surgery lasts for at least 3 hours, the intervention period fluid administration according to protocol will last 10-12 hours, the study period will be 20-24 hours. Blood samples for analysis of plasma sodium, potassium, glucose and osmolality are collected preoperatively and the following morning together with analysis of urine produced during the study period. 