To compare the hemodynamics and anesthetic effects of spinal tetracaine containing either clonidine or phenylephrine administered to normotensive or hypertensive patients, we studied 75 patients allocated to one of three groups, each of which included nine hypertensive patients: clonidine group (n = 25) received 2 mL of 0.5% tetracaine (10 mg) containing clonidine (0.15 mg); phenylephrine group (n = 25) received 2 mL of 0.5% tetracaine (10 mg) containing phenylephrine (3 mg); and control group (n = 25) received 2 mL of 0.5% tetracaine (10 mg) alone. Analgesic levels and the intensity of motor block obtained did not differ significantly among the three groups. However, the regression of sensory block (skin dermatome) was significantly slower in patients given clonidine (T9 ± 3) or phenylephrine (T9 ± 3) at 300 min after the injection than in those (L1 ± 3) in the control group (P < 0.01). Between normotensive and hypertensive patients, there was no significant difference in sensory and motor block in the three groups. Mean blood pressure (MBP) in both hypertensive and normotensive patients given tetracaine-clonidine remained approximately 20% lower than that in the control group for as long as 7 h after the injection (P < 0.05). We conclude that the inclusion of clonidine can provide similar effects to that of phenylephrine with respect to prolongation of hyperbaric tetracaine spinal anesthesia, but could cause more hypotension, without bradycardia, in both normotensive and hypertensive patients for a prolonged time (i.e., 420 min).