Background:Spinal anesthesia is a common choice for patients undergoing orthopedic surgeries.ondansetron was found to attenuate the incidence of SIH and bradycardia during spinal anesthesia. Aim of the study:This work was done to compare between the effects of prophylactic intravenous of two different doses of ondansetron (4 mg) and (8 mg) in attenuating hypotension in patients undergoing orthopedic surgeries under spinal anesthesia. Patients and methods:This study wasa prospective comparative randomized controlled clinical trial that have been carried out in Zagazig University Hospitals and included 66 patients, their ages ranged from 21 to 60 years old patients for only unilateral orthopedic surgeries under spinal anesthesia, duration of surgery less than 2 hours in the study.Patients were randomly divided into three equal groups, 22 patients for each group, Group "O1" received IV ondansetron (4mg) diluted in 10 ml saline, group "O2" received IV ondansetron (8mg) diluted in 10 ml saline, and group C (control) received only 10 ml IV saline alone. Medications were administrated 5 min before starting the subarachnoid block by an anesthesiologist blinded to them, they were assessed for their cardiovascular effects including blood pressure, heart rate (HR) before, throughout and after operation. Results:There was difference between the prophylactic intra venous of the two different doses of ondansetron in attenuating hypotension in patients undergoing orthopedic surgeries, under spinal anesthesia compared with the control group. Mean Blood Pressure (MBP) and HR were significantly, lower among the control group from 5 min tell the 10 minutes compared to (O1) and (O2) groups. Conclusion: In patient undergoing orthopedic surgeries under spinal anesthesia, prophylactic intravenous administration of 4mg ondansetron or 8mg ondansetron 5min before induction of spinal anesthesia to reduce the severity of spinal-induced hypotension and bradycardia well significantly.