BACKGROUND: Minimum effective anesthetic concentration (MEAC) of local anesthetics for spinal anesthesia, defined as the concentration at which a spinal anesthetic agent produces surgical anesthesia within 20 minutes of administration in 50% of patients. This concept is similar to the concept of minimum alveolar concentration (MAC) for inhalation anesthetic. Furthermore, as MEAC may vary with the administered dose, it will be determined using 10 mg of hyperbaric tetracaine. METHODS: 10 mg hyperbaric tetracaine containing dextrose 10 % was administered intrathecally to 12 patients (ASA I-II, age 20~40 yr), who were undergoing lower limb or urological procedures within 90 minutes, at concentrations ranging from 0.07~0.1%. The choice of tetracaine concentration was determined by Dixon's up-and-down method. Complete anesthesia was defined as: (1) pinprick anesthesia at or higher than T10 and (2) complete leg paralysis; all occurring in both lower extremities within 20 min. General anesthesia was initiated if spinal anesthesia was incomplete. RESULTS: MEAC of 10 mg hyperbaric tetracaine was 0.083 %. Mean anesthetic duration was 75.2 minutes (range: 40~100 min). The maximum sensory block level ranged from T3~T8. CONCLUSION: The aim of this study was to establish new concept of minimum effective anesthetic concentration of hyperbaric tetracaine for spinal anesthesia. We recognized that spinal anesthesia can be accomplished with very dilute solution. As fixed dose, no correlation was found between concentration of the spinal tetracaine solution with the highest sensory level, anesthetic duration and the degree of mean arterial pressure decreased.