To define the effect of alkalinization of bupivacaine 0.5% in supraclavicular approch of brachial plexus bock, the onset of sensory and motorblock were determined. Fourty physical status ASA 1 were randomly allocated to two groups.: Group 1 (n=20); bupivacaine 0.5% 20 ml (pH 6.0-6.2). Group 2 (n=20); alkalinized buivacaine 0.5% 20 ml (pH 6.9-7.2). Onest of sensory blockade were determined by pinprick in the C4 - T2 skin dermatome, and extend of sensory block was assessed by the number of dermatomes blocked while motor blockade was assessed by scoring on a scale: Grade 1; inability to flex the elbow against resistance. Grade 2; inability to flex the elbow against gravity. Grade 3; inability to flex the wrist against gavity. The results were as follows. 1) The average time for sensory blockade of five dermatonies was significantly more rapid in group 2 (within 15 min) than those in group 1 (over 25 min). 2) The average time of motor blockade was significantly more rapid in group 2 (Grade 1: 2 min 43 sec, Grade 2: 11 min 36 sec) than those in group 1 (Grade 1: 5 min 4 sec, Grade 2: 18 min 36 sec). 3) There was no pneumothorax, phrenic nerve paralvsis and general seizure or other side effects but Horners syndrome in 10 cases and hematoma in 2 cases were observed. The results indicate that alkainized bupivacaine for supraclavicular approch of rachial plexus block has more rapid onset than plain bupivacaine.