The aim of this study is to evaluate the effect of N2O on the required dose of propofol in creating the perfect sedation for painful procedures in children. The first, 88 children who are candidates for bone marrow aspiration, are randomly divided into 2 groups. In Group I, combination of midazolam 0/03 mg / kg and fentanyl 1 μg / kg and ketamine 1 mg / kg and propofol 1 mg / kg are injecting intravenously. In Group II ,combination of midazolam 0/03 mg / kg and fentanyl 1 μg / kg and ketamine 1 mg / kg and propofol 1 mg / kg are injecting intravenously and from 2 minutes before the start of the procedure until the end of the procedure the patient is used to ventilate the gas entonox. In both groups, the aim is achieve to sedation Level 4 is based on Ramsay sedation scale, if necessary, every 1 minute ,propofol 1 mg / kg to be administered intravenously. The SpO2 and HR record before drug administration, immediately before the start of the procedure, at the beginning of the procedure, at the end of the procedure. The satisfaction will be evaluated and documented, by the physician performing the procedure and help nurse him by VAS (0-10). In addition to ,age, weight, duration of procedure, recovery time, the prescription drugs for each patient and the occurrence of any complication record.