BACKGROUND: Accurate identifying and assessment of the degree of pain that the animal is suffering can be a challenge, and, control of painful condition is becoming an increasingly important part of veterinary medicine. OBJECTIVES: This study was carried out to compare different tools for postoperative pain assessment in bitches. METHODS: Ten adult mixed breed bitches were selected and randomly divided into two equal treatment and control groups. Anaesthesia was premedicated with acepromazine (0.03 mg/kg, IM) and induced with Sodium thiopental (6-10 mg/kg, IV). Halothane was used for maintenance of the anesthesia. Ovariohysterectomy performed in the two groups. Treatment group received 3 mg/kg of tramadol intramuscularly (i.m.) and control group received normal saline (equal volume with tramadol, i.m.) before the anesthetic induction. After operation the injections of tramadol and normal saline were repeated for every 6 hours in 7 days. The animals were monitored at hour 2, 3 and 4 after each injection and they were scored for signs of pain by two trained assessors who were blinded to the groups. The measured variables were pain assessment with different methods including Simple Descriptive Scale (SDS), Visual Analogue Scale (VAS), and University Melbourne Pain Scale (UMPS). Duration of anesthesia and duration of surgery, were also recorded. RESULTS: There were no significant differences between the two groups in regard to analgesia that were measured based on VAS and SDS methods, but in UMPS method, analgesia was significantly better in treatment group. Among simple clinical criteria body temperature and respiratory rate did not show any significant alterations, but heart rate had significant changes between the groups. CONCLUSIONS: The ability to quantify the degree of pain experienced by animals is an important aspect in the assessment of animal welfare; in addition, we concluded, that the great challenge for the veterinarians is the evaluation of postoperative pain in dogs