Increasing the pH of local anesthetic solution with sodium bicarbonate has been known to hasten its onset of action. This study was designed to verify the effect of pH adjustment of bupivacaine on caudal anesthesia. Forty adult male patients having caudal block for perianal surgery were randomly assigned to four groups(n=l0) as follows. Group 1: control, 0.5% bupivacaine 20ml+0.6ml saline Group 2: 0.5% bupivacaine 20 ml+0.1% epinephrine 0.1 ml+0.5ml saline Group 3: 0.5 bupivacaine 20ml+5% sodium bicarbonate 0.5ml+0.1 ml saline Group 4: 0.5% bupivacaine 20 ml+5% sodium bicarbonate 0.5ml+0.1% epinephrine 0.1 ml The pH of prepared solution of each group is 6.23±0.032, 5.95±0.028, 7.35±0.054, 6.92±0.067 respectively. The time of onset of anesthesia (time between the completion of anesthetic injection and loss of temperature sensation at S4, S5 dermatome) was significantly rapid in group 3 followed by group 4. The time of onset of surgical anesthesia (time between the completion of anesthetic injection and loss of pain sensation by pin-prick test at S4, S5 dermatome) was significantly rapid in group 3.