The clinical effects of equal hyperbaric doses of bupivacaine HCl and tetracaine HCl were studied and compared in 99 adult men undergoing spinal anesthesia for similar surgical procedures. The spinal anesthetic agents were used in random order in various dosages. Comparisons were made of (1) amount of anesthetic agent used, (2) elapsed time for onset of analgesia, (3) elapsed time for obtaining maximum anesthesia, (4) cephalad dermatome level of anesthesia, (5) incidence of motor blockade, and (6) duration of anesthesia. Results with the 2 drugs in equal dose, volume, and specific gravity were almost identical. However, tetracaine provided a motor block in a significantly greater number of patients than did pubivacaine (100 versus 42%, rho less than 0.001) when both drugs were used at the 9.75-mg dose level.