Objective. Total knee replacement is one of the most painful orthopedic surgical procedures. In this study, our goal was to compare the intraoperative and postoperative hemodynamic effects, the side effects, the effect on the duration of pain start, the 24-hour VAS, and the amount of additional analgesia used, of the fentanyl and morphine we added to the local anesthetic in the spinal anesthesia we administered in cases of elective knee replacement. Materials and Methods. After obtaining the approval of the Erciyes University Medical Faculty Clinical Drug Trials Ethics Committee, as well as the verbal and written consent of the patients, we included 50 patients in our prospective, randomized study. Results. In our study, the morphine group (Group M) had lower pain scores in the 2nd, 6th, 12th, and 24th hours compared to the fentanyl group (Group F). When additional analgesic requirements were compared, it was found that in the 2nd, 6th, and 24th hours fewer Group M patients needed more analgesics than did Group F patients. Conclusion. The fentanyl group also had lower first analgesic requirement times than did the morphine group. In terms of nausea and vomiting, there was no statistically significant difference between the two groups.