Epidural analgesia was administered to one hundred patients undergoing various types of surgical procedures. They were divided at random into four equal groups who received lidocaine hydrocarbonate or lidocaine hydrochloride, both with or without epinephrine. A double blind method was used. The study was designed first to test the validity of claims that lidocaine hydrocarbonate produces a shorter period of onset for effective analgesia, a more profound sensory and motor block, and a higher spread of analgesia than the hydrochloride salt and secondly, to identify the respective roles of carbon dioxide and epinephrine in obtaining this alleged superior effectiveness. The results of the study showed that carbon dioxide improved the quality of sensory block, but we could not find any significant difference between lidocaine hydrocarbonate and lidocaine hydrochloride salt, with and without epinephrine, with regard to rapidity of onset, upward spread of analgesia and quality of motor block. As was already known, duration of analgesia was prolonged by the addition of epinephrine but not by the addition of carbon dioxide. They study also showed that the compliance of the epidural space was decreased in the lidocaine hydrocarbonate groups compared to those with lidocaine hydrochloride. There is a positive correlation between the duration of sensory block or the upper level of analgesia and compliance in the hundred patients studied. It is concluded that the hydrocarbonate base, because of its more profound sensory block in the L5-S1 segment, can be useful for operations on the lower extremities, especially in the L5-S1 segmental distribution. However, knowing that the hydrocarbonate base is more expensive, one must use his own judgment in appraising the cost-benefit of its use.