The patients are randomly divided into two groups according to the last digit of their 9-digit identification number: patients with an odd number are allocated to Sevoflurane (SEVO group) and patients with an even number are allocated to and Isoflurane (ISO group). Preanesthetic medication consists of midazolam given either orally (0.5 mg/kg up to a maximum of 10 mg) or intravenously (2.0 mg) when intravenous (IV) access is established before induction of anesthesia. Anesthesia is induced either via facemask with Sevoflurane (Abbot Laboratories Ltd., UK) in 66% nitrous oxide and 33% oxygen or IV with Propofol (Diprivan, B Braun, Germany) 2.5 mg/kg. In patients who undergo mask induction intravenous line is established after losing the consciousness, Sevoflurane is discontinued and Propofol (2.5 mg/kg) is administerd IV. Before tracheal intubation the lungs will be preoxigenated with 100% O2. Pancuronium (0.1 mg/kg) and Fentanyl (2-4 µg/kg) will be used to facilitate tracheal intubation. After tracheal intubation, the fresh gas flow will be set to 1 l/min O2 and 1 l/min room air for the remainder of the procedure. Thereafter, anesthesia will be maintained with the assigned study drug (SEVO or ISO) in an oxygen/air 50% mixture. In the ISO group, the Isoflurane will be started with an inspiratory fraction of 1MAC and in the SEVO group the Sevoflurane will be started with an inspiratory fraction of 1MAC adjusted for age. Pancuronium is used as the neuromuscular drug during the intraoperative period, as needed.