1) Objectives :Clinical Trial of The Effect of Low Dose Ketamine on Blood Pressure During Positioning After Induction of Anesthesia in Patients Undergoing Lumbar Discectomy ,Clinical Trial of The Effect of Placebo on Blood Pressure During Positioning After Induction of Anesthesia in Patients Undergoing Lumbar Discectomy , Comparison of the Blood Pressure Reduction after Induction of Anesthesia in Two Groups 2) Design: Patients Undergoing Lumbar Discectomy , in Two Groups Ketamine and Placebo , 42 Patient in each Group , Double Blind , Single Center, Recruiting, Randomisation by Random number tables, 3) Setting and conduct: Induction in All Patients will be simillar including Midazolam 0.03 mg/kg, Fentanyl 2 micg/kg, Propofol 1 to 2.5 mg/kg and Atracurium 0.5 mg/kg . After Induction and Intubation, in Ketamine Group 0.5 mg/kg and in Placebo Group Normal Saline with the same volume will be injected . After 90 Seconds to reach the Peak Effect of Ketamine All the Patients with a similar technique will be Positioned. Systolic , Diastolic and Mean Arterial Blood Pressure will be recorded Preoperative , Pre Intubation , one minute after Intubation , before Positioning , Immediately after Positioning the Patient from Supine to Prone Position , then every 2 minute until 10 minutes after Positioning . In the Case of Hypotention more than 20 percent Ephedrine 5 mg ( Heart Rate Less than 100/min ) or Phenylephrine 50 to 100 microgram ( Heart Rate more than 100/min ) will be used . 4) Participants Inclusion Criteria: Patients candidated for Lumbar Discectomy, Age between 20 to 40 year with ASA(American Society of Anesthesiologists) Class I, Exclusion Criteria: History of Cardiovascular Drugs usage, Emergency Procedures, Body Mass Index more than 40. 5) Intervention: 1- Midazolam 0.03 mg/kg, Fentanyl 2 micg/kg, Propofol 1 to 2.5 mg/kg and Atracurium 0.5 mg/kg and Endotracheal Intubation , Ketamine 0.5 mg/kg 1.5 minutes before Positioning The Patient from Supine to Prone Position , 2- Midazolam 0.03 mg/kg, Fentanyl 2 micg/kg, Propofol 1 to 2.5 mg/kg and Atracurium 0.5 mg/kg and Endotracheal Intubation , Placebo with the same volume (Normal Saline ) 1.5 minutes before Positioning The Patient from Supine to Prone Position , 6) main outcome measures: Blood Pressure changes