The effects of deliberate hypotension by sodium nitroprusside (SNP) on the cardiovascular system and dosage of SNP were studied in 10 patients undergone Cotrel-Dubousset instrumentation for the operative treatment of scoliosis, spinal stenosis or herniated intervertebral disc. Mean arterial pressure decreased significantly (p<0.05), but cardiac output, central venous pressure and sytemic vascular resistance showed no significant change, The correlations between average dose of SNP and age, body weight, and age to weight ratio were r= -0.4 (p>0.05), r= -0.2 (p>0.05), and r= - 0.25 (p>0.05) respectively. The amount of whole blood transfused during the operation was 5. 3±1 .7 units. Hemoglobin and hematocrit decreased significantly (p<0.05) from 11.3±1.3g/dl and 33.7±4.1% to 10.1±1.5g/dl and 30±4.1% during operation.