BACKGREOUND: Tetracaine has been the most commonly used spinal anesthetic agent. Recently bupivacaine has been found to be a useful agent, and has been reported not to cause neurologic complications as well as being a better quality anesthetic. The purpose of this study was to compare the anesthetic effects of bupivacaine with that of tetracaine. METHODS: Forty patients undergoing lower abdomen and lower limb operation were randomly allocated to two groups. Group I (n=20) received 15mg, 0.5% tetracaine in 10% dextrose, while Group II (n=20) received 15mg, 0.5% bupivacaine in 8% dextrose. After intrathecal injection of the agent at the level of L3-4 intervertebral space, the changes in sensory and motor block, blood pressure and heart rate were measured in two groups. RESULTS: The onset time of sensory block was more rapid in Group I than in Group II. Two segment regression time of sensory block was significantly longer in Group II than in Group I (p<0.05). The time for maximum motor block was shorter in group I than in Group II (p<0.05), but the duration of decreased motor block was significantly longer in Group I. The change of systolic blood pressure in Group II was less than the change in Group I. CONCLUSIONS: Bupivacaine has a longer sensory block duration, a weaker, shorter motor block duration and yields a less change in systolic blood pressure than tetracaine in spinal anesthesia.