Background: There were several studies using either low dose Bupivacaine as unilateral selective spinal anesthesia (SSA) or low dose lidocaine SSA for ambulatory knee arthroscopy. There were many concerns about high failure rate, complications, and different times to home readiness discharges. Aim of the Study: The study aimed to: (1) Compare the clinical effi cacy and side effects of two different techniques of SSA in subarachnoid block for ambulatory knee arthroscopy (2) evaluate the possibility of a shorter stay in post-anesthesia care unit (PACU) or eligibility for fast tracking anesthesia (bypassing PACU) after SSA and numbers of patient bypassed PACU (3) compare the time to ambulate and time to home readiness. Patients and Methods: Prospective, randomized and open study was conducted, including 50 outpatients undergoing knee arthroscopy. Patients allocated into two groups: Bupivacaine group (group B); was injected with 3 mg bupivacaine and 10 ug fentanyl intrathecally in lateral decubitus position and remained for 20 min then supine position. Lidocaine group (group L) was injected with 20 mg lidocaine, plus 25 ug fentanyl intrathecally and immediately keeps in the supine position after injection. The quality and durations of motor and sensory block were compared between groups. Sensory block assessed by pin brick method and motor block assessed by Bromage scale. Time spent in PACU, the time to ambulate, and times to home-readiness were compared. Number of patients bypassed PACU was recorded. Side effects as pain, nausea, vomiting, postdural puncture headache, transient neurological symptoms, retention of urine and pruritus were evaluated and compared after SSA. Results: Bupivacaine group provided unilateral spinal anesthesia with signifi cantly longer duration of both motor and sensory blocks than bilateral spinal of lidocaine group P < 0.001 and P < 0.01 respectively. The upper dermatomal height of sensory block reached was T9 and T8 in groups B and L respectively. Group B showed more intense motor block than group L. There was no signifi cant difference between both groups as regards postoperative side effects P > 0.05. Group L patients did not stay in PACU, and all patients bypassed it P < 0.001. Bupivacaine group patients showed signifi cantly longer time to ambulate and time to home-readiness than lidocaine group P < 0.001 and P < 0.01 respectively. Conclusion: Both techniques of SSA were clinically effi cient as subarachnoid block,