It is essential to identify objective and efficient methods of evaluating postoperative pain in rodents. The authors investigated whether postoperative changes in rates of body weight gain could serve as a measure of the efficacy of meloxicam or buprenorphine analgesia in growing rats. Young adult male Lewis rats underwent general endotracheal anesthesia and thoracotomy and were treated postoperatively for 3 d with saline (no analgesia), buprenorphine (six doses of 0.1 mg per kg) or meloxicam (three doses of 1 mg per kg). The authors evaluated rats' daily growth rates for 5 d after surgery and compared them with baseline (preoperative) growth rates. To discriminate between the effects of postoperative pain and other concurrent physiologic effects associated with anesthesia, thoracotomy or analgesia, the authors evaluated weight changes in multiple control groups. Treatment with buprenorphine in the absence of any other procedure or with anesthesia alone significantly affected rats' body weight. Notably, growth rate was maintained at near normal levels in rats treated postoperatively with meloxicam. These findings suggest that growth rate might serve as an efficient index of postoperative pain after major surgical procedures in young adult rats treated with meloxicam but not in rats treated with buprenorphine.