BACKGROUND: In anesthesia, clonidine has been proven to be clinically useful in producing sedation, anxiolysis, and decreasing the requirement for both general and regional anesthetics. Studies have shown that oral clonidine is able to prolong the duration of spinal anesthesia when bupivacaine or isobaric tetracaine is used. This study was conducted to determine the effects of oral clonidine premedication on the duration of hyperbaric tetracaine spinal anesthesia. METHODS: Forty-six patients for lower extremity and perineal surgery under hyperbaric tetracaine spinal anesthesia were randomly grouped into two: group I - control and group II - clonidine. The clonidine group received 150 mcg oral clonidine premedication 1 hour prior to the spinal anesthesia. Sensory level was measured using the pinprick method. The onset, maximal sensory block, duration and the hemodynamic variables were determined and compared between the two groups. RESULTS: No significant differences were noted as to onset and the maximal level of sensory blockade. The mean duration of sensory analgesia was significantly prolonged in the clonidine group (298.33 min) as compared to the control group (176.73 min). Hemodynamic variables were significantly decreased in the clonidine group. CONCLUSION: This study demonstrated that 150 mcg oral clonidine given 1 hour prior to tetracaine spinal anesthesia significantly prolonged the duration of the blockade.