BACKGROUND: Ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, is known to have analgesic properties in subanesthetic doses and has been used as an analgesic in the postoperative period by variable routes. The effect of adding ketamine to analgesia using intravenous PCA morphine was evaluated in 90 women after cesarean section. METHODS: Ninety parturients were randomly allocated to three groups and each group had 30 women. The parturients in group 1 were given analgesics of morphine only, group 2 were given analgesics of the 2 : 1 mixture of morphine and ketamine, and group 3 were given analgesics of the 1 : 1 mixture of morphine and ketamine. We evaluated the analgesic requirement, numerical rating pain score, side effects and patient's satisfaction. RESULTS: The morphine requirement in group 3 was significantly lower than that in groups 1 and 2 at 3, 6, 12, 24 and 48 hours postoperatively. The pain score in group 2 was lower than that in group 1 at 3 and 6 hours and the pain score in group 3 was lowest of all groups at 3 and 6 hours. The incidence of dizziness was higher in group 3 than in groups 1 or 2. CONCLUSIONS: We concluded that adding ketamine with morphine in using an intravenous PCA can decrease analgesic requirements and improve analgesic property.