Sodium nitroprusside(SNP) is used to induce hypotension for a wide variety of indica- tions. Ordinarily, blood pressure responds sensitively to infusion of SNP in low doses, but occasioally resistance is seen, and actual tachyphylaxis during SNP infusion has been reported. To investigste the continuous infusion rates of SNP, we retrospectively reviewed 144 cases of spinal fusion operations which had been performed under deliberate hypotensive anesthesia (mean arterial pressure at 50-60 mmHg). To produce deliberate hypotension, The mean dose of SNP was 17.16 mg, the mean infusion time 283.85 minutes, and the average infusion rates 1.05 ug/kg/min. Patients who received csptopril required less SNP than untreated patients(0.95 vs 1.23 ug /kg/min., p<0.05). Isovolemic hemodilution also reduced aversge infusion rates of SNP (0. 87 vs 1.22ug/kg/min., p<0.05). There were, however, no significant differences in preoperative hypertention vs normotension, mild hypothermia vs. normothermia during the operation, and male vs. female. In addition, the average infusion rates of SNP were significantly correlated with body mass index(r=0.3329, p<0.01). But those were not correlated with age, infusion time of SNP, weight, volume of transfusion, height/age, and height.