Patient controlled analgesia (PCA) by intravenous pentazocine was performed to determine its efficacy and the dose required for the pain relief after gynecological or obstetric operations. After obtaining informed consent, studies were performed on 28 female patients (ASA I, II: Mean age 38.1 years: Mean weight, 53.8 kg) who had received gynecological or obstetric operations with lower abdominal incision. Anesthesia given was nitrous oxide and isoflurane combined with epidural anesthesia with 1% mepivacaine used only during the operation. Six patients had cesarian section under spinal anesthesia. No patients received opioid during anesthesia. PCA was performed with a Graseby PCA pump. Lockout time was 8 minutes and the bolus dose was 3 mg. In all the patients, satisfactory pain relief was obtained and no other analgesic was necessary. Mean initial dose was 169.4 micrograms.kg-1 and the mean doses used for following each 6 hours until 24 hours were 409.7, 368.6, 279.3 and 211.1 micrograms.kg-1 respectively. Evaluation of PCA by the patients after the procedure showed excellent (13 patients) good (12) and passable (3) analgesia. No significant complication was observed except temporary nausea in two patients. Satisfactory postoperative pain relief could be obtained by relatively small doses of pentazocine and adverse reactions related especially to sigma receptor could be avoided.