The objective of this study was to compare the effects of postoperative pain following ovariohysterectomy via harmonic scalpel assisted laparoscopy (HALO) and traditional ovariohysterectomy (OVH) in dogs. The study was designed as a randomized, blinded, prospective study. Sixteen, purpose-bred, intact female, Beagle dogs were used to complete the study. Dogs were placed into two groups. Group 1 included (8 dogs) that underwent ovariohysterectomy via HALO. Group 2 included (8 dogs) that underwent ovariohysterectomy via traditional OVH. Physiologic data, abdominal nociceptive threshold scores, and University of Melbourne pain scores (UMPS) were recorded at 2, 6, 12, 24, 48, and 72 hours following surgery. Blood samples for plasma cortisol, glucose, and creatine phosphokinase (CPK) were taken at the time of the incision and 2, 6, 12, 24, 48, and 72 hours following surgery. No significant surgical complications were encountered in either group. The HALO mean surgical time was significantly longer (55.7 minutes) than the traditional OVH (31.7 minutes). No significant differences were observed between the two groups for the pain measures of heart rate, respiratory rate, temperature, CPK, and glucose. The OVH group had significantly higher mean plasma cortisol levels at hour 2 following surgery than the HALO group (P=0.0001). The mean UMPS were significantly higher in the OVH group than the HALO group at all postoperative times (P=0.0001). Mean nociceptive threshold measurements revealed significantly higher tolerated palpation pressures in the HALO than the OVH group at all postoperative times, except hour 72 (P=0.0002). Dogs in this study appeared to be less painful with HALO procedures versus traditional OVH. The harmonic scalpel coagulated ovarian and uterine vessels completely with minimal collateral damage to surrounding tissues. The clinical relevance of this study demonstrates that harmonic scalpel-assisted laparoscopic ovariohysterectomy is a safe alternative to traditional OVH and offers a minimally invasive and less painful method of surgery. / Master of Science