Effective doses of epidural morphine for relief of postcholecystectomy pain. Having previously established the effective dose of intrathecal morphine for relief of postcholecystectomy pain, we determined in this study the effective dose of epidural morphine for relief of postcholecystectomy pain in 154 patients given epidural injections of a placebo (group 1, n = 49), 2 mg morphine (group 2, n = 54), or 4 mg morphine (group 3, n = 51) intraoperatively mixed in 1.5% lidocaine. The percentage of patients who did not request an analgesic, 30 mg IM pentazocine, for relief of pain during the first 24 postoperative hours was significantly greater in groups 2 and 3 than in group 1. In patients who did need 30 mg IM pentazocine postoperatively, the number of times pentazocine was administered was also significantly greater in group 1 than in groups 2 and 3. The percentage of patients developing respiratory depression or vomiting in the first 48 postoperative hours was similar in the three groups. Based on the present data and those we previously reported for intrathecal morphine, we conclude that an epidural morphine dose of 2-4 mg and an intrathecal morphine dose of 0.06-0.12 mg are equipotent for relief of postcholecystectomy pain. 