Increasing the pH of 2% chloroprocaine (CP) with sodium bicarbonate to pH 7.7 hastens the onset of epidural analgesia. The purpose of the present study was to determine the effect of pCO2 on the onset of epidural analgesia with CP buffered to a constant pH 7.7. Four groups consisting of ten patients each were studied: C, control, commercial CP (pH 4.35); B, CP buffered with sodium bicarbonate (pH 7.7); T, CP buffered with tromethamine (pH 7.7), and BT, CP buffered with sodium bicarbonate and tromethamine (pH 7.7). All epidural catheters were placed at the L2-3 or L3-4 interspace using a loss-of-resistance technique with air with each patient in a sitting position. The pH and pCO2 of each local anesthetic solution were measured as well as the onset and duration of analgesia. The study groups did not differ with respect to demography or entry characteristics. There were intergroup differences in the pCO2 values of the study solutions as follows: Group 1 (C), 11.8 +/- 1.5 mmHg; 2 (B), 113.0 +/- 1.4 mmHg; 3 (T), 3.0 +/- 0.3 mmHg, and 4 (BT), 74.1 +/- 1.0 mmHg, respectively. The time to the onset of analgesia was significantly faster in Group 2 (B; 2.7 +/- 0.8 minute), while the onset of analgesia was significantly slower for Group 3 (T; 5.4 +/- 0.4 minute) than either Group 1 (C; 4.2 +/- 0.8 minute) or Group 4 (BT; 3.4 +/- 0.3 minute). Regression analysis revealed that the onset times of the buffered solutions were significantly related to pCO2 (r2 = 0.81).(ABSTRACT TRUNCATED AT 250 WORDS)