Prolonged motor block and delayed ability to walk are limitations of spinal anesthesia in ambulatory (same-day) surgery. This can be improved by lowering the dose of local anesthetic (a medication that, when injected around nerves, blocks nerve conduction, resulting in numbness and weakness) used in the spine, but too low a dose can result in an incomplete block (inadequate anesthesia) in some patients. There is evidence that adding a low dose of fentanyl, a narcotic, to mepivacaine enhances the anesthetic effect. The purpose of this study is to determine the lowest dose of mepivacaine, a local anesthetic, when combined with fentanyl, for which spinal anesthesia is adequate for ambulatory knee arthroscopy.