BACKGROUND The effect of different injection temperatures on carbonated anesthetics has been controversial since 1965. The current study was undertaken to determine onset times of sensory and motor blockade after epidural anesthesia with 0.5% bupivacaine-CO2 and 0.5% bupivacaine-HCl at various injection temperatures. METHODS The study was performed on 90 ASA class I-II urologic patients during extracorporeal shock wave lithotripsy. The patients were randomized in six groups to receive either 0.5% bupivacaine-CO2 or 0.5% bupivacaine-HCl at temperatures of 4 degrees, 20 degrees, or 36 degrees C. The six groups were comparable in age, height, and weight. Epidural anesthesia was performed at the L2-3 interspace with an 18-gauge Tuohy needle using loss of resistance. A catheter was advanced 4 cm in the epidural space and 4 ml 0.5% bupivacaine with adrenaline 1:200,000 was given as a test dose. After 4 min the full anesthetic dose, based on body size, was injected with the patient supine. Sensory blockade was determined by the pinprick method and motor blockade by the Bromage method at 2-min intervals for the first 20 min, at 5-min intervals for the next 10 min, and then every 15 min to a total of 240 min. Statistical analysis was done by the Mann-Whitney test, with P less than 0.05 considered significant. RESULTS Spread of sensory blockade was significantly faster with bupivacaine-CO2 and -HCl at a temperature of 36 degrees C as compared to 4 degrees or 20 degrees C (P less than 0.05) (Figs. 1, 2 and Table 2).(ABSTRACT TRUNCATED AT 250 WORDS)