60 patients undergoing elective shoulder surgery and receiving an interscalene nerve block were randomized to an injection of 0.5% ropivacaine either directly inside the brachial plexus sheath, or outside the brachial plexus sheath. The study was conducted in order to determine which administration method yielded a faster nerve block and to assess the side effects of each injection technique. It was found that injecting the local anesthetic into the brachial plexus sheath resulted in a faster nerve block onset; however, it also resulted in an increased incidence of transient paresthesia while positioning the needle, as compared to injecting outside the sheath. All other outcomes were comparable between groups.