This study reports the results of a double-blind, parallel-group comparison of intravenous methadone with morphine for the control of postoperative pain. Twenty patients (ASA Status 1 or 2) undergoing a surgical procedure involving an upper abdominal incision were randomly allocated to the methadone (n = 10) or morphine (n = 10) treatment groups. The patients were administered a 20-mg intraoperative opioid dose and 5-mg intravenous increments of opioid from precoded syringes in response to pain in the recovery and surgical wards. There was no significant difference between the mean +/- SD amount of supplementary methadone (8 +/- 6.3 mg) and morphine (9 +/- 9 mg) required in the recovery ward to provide initial pain control. The time from initial pain control to the first supplementary dose in the surgical ward was significantly different (P less than 0.01) in the methadone group (20.7 +/- 20.2 h) when compared to the morphine group (6.2 +/- 3.0 h). Further, patients required significantly less (P less than 0.001) methadone (11.5 +/- 8.5 mg) than morphine (41 +/- 14.1 mg) in the surgical ward to provide adequate pain relief throughout the duration of the study (i.e., 60 h). There was a significant difference in visual analogue pain scores between the methoadone and morphine groups on postoperative days 1 and 2, suggesting the quality of pain relief was similar for both treatment groups. Blood opioid-concentration monitoring indicated that there was a relationship between blood opioid concentration and pain relief.(ABSTRACT TRUNCATED AT 250 WORDS)