BACKGROUND: Narcotic drugs are usually used for postoperative pain control which could cause several complications such as respiratory depression and apnea. Therefore, replacement of these drugs with safer analgesics is recommended. The aim of this study was to compare the analgesic effects of suppository diclofenac and pethidine in post-cesarean section (C/S) patients. METHODS: In this clinical trial, pregnant women who were admitted to Alzahrah Hospital, Isfahan, Iran, between August and February 2004 and met the inclusion criteria were recruited. After obtaining informed consent, the patients were randomly assigned (block randomization) to two groups. In group A, 100 mg rectal suppository diclofenac was used after operation at four time points: at the end of operation and 8, 16 and 24 hours after the operation. Group B received pethidine 1 mg/kg intramuscularly at similar time points. The pain scores were assessed at 2, 10, 18 and 26 hours after C/S using the Visual Analogue Scale (VAS). Age, gestational age, parity, history of previous abortion, C/S and abdominal surgery, level of education, pain score, side effects and satisfaction level were assessed. Analysis was carried out with ANCOVA model and χ2 Mantel Haenszel tests by SPSS.10 software. P<0.05 was considered significant. RESULTS: Two hundred forty patients met inclusion criteria. Age, parity, history of previous abortion, history of previous C/S and abdominal surgery, level of education and satisfaction level were similar in the two groups (P>0.05). There was no significant difference between side effects in the two groups except for dizziness in 11 cases in group B. There were significant statistical difference between pain intensity in 10, 18 and 26 hours after C/S in group A and group B (2.05 ± 2.07, 1.4 ± 1.6 and 0.5 ± 1.1 vs. 2.6 ± 2.2, 2.3 ± 2.2 and 1.3 ± 1.9) respectively. CONCLUSIONS: The results of the present study showed that the use of suppository diclofenac is an appropriate replacement therapy for pain relief after C/S. KEY WORDS: Diclofenac, pethidine, cesarean section, pain relief.