BACKGROUND: Although spinal anesthesia has a lot of advantages, it has some disadvantages or undesirable effects. Hypotension and unnecessarily long neural blockade are included among them. Although using small dose local anesthetics fairly solves these problems, it is insufficient to provide reliable surgical anesthesia by itself. Therefore the authors investigated whether such an opioid as fentanyl and a small dose local anesthetic used together during spinal anesthesia can prevent hypotension and unnecessarily long neural blockade and provide reliable surgical anesthesia simultaneously. METHODS: Thirty patients undergoing knee or below knee surgery were randomized into two groups. Group 1 received bupivacaine 5 mg combined with fentanyl 20 micro gram, and group 2 received 10 mg bupivacaine. Hypotension was recorded and was treated with intravenous ephedrine. Sensory blockade, intraoperative analgesia, motor blockade and side effects were assessed. RESULTS: No significant differences were observed in values for assessing hypotension, sensory blockade or intraoperative analgesia between the two groups. Also no significant differences were observed in intensity of the motor blockade and side effects between the two groups. However the duration of the motor blockade of group 1 was longer significantly than that of group 2. CONCLUSIONS: Small dose bupivacaine and fentanyl administered together intrathecally reduced duration of motor blockade and didn't augment of side effects and provided reliable anesthesia for surgery of knee or below knee simultaneously.