The effect of induced hypotension with sodium nitroprusside(SNP) on alveolar-arterial oxygen tension difference and pulmonary shunting in 10 surgical patients with normal pulmonary function was studied under general anesthesia. Heart rate, mean arterial pressure, central venous pressure, blood gases and hemoglobin were measured before, during and after SNP infusion. Using the above data, alveolar-arterial oxygen tension difference and pulmonary shunting were calculated. The results were as follows. 1) Mean arterial pressure decreased significantly from 98+/-9.1mmHg before SNP infusion to 61+/-3.0 mmHg during SNP infusion(p<0.001), and increased to 96+/-6.7mmHg again after SNP infusion. 2) Heart rate increased significantly from 78+/-10.5 beats/min. to 101+/-11.9 beats/min.(p<0.001), and decreased to 84+/-11.5 beats/min. again. 3) Arterial oxygen tension decreased from 489+/-32.5mmHg to 480+/-25.0mmHg, but was not statistically significant. (p>0.05). 4) Alveolar-arterial oxygen tension difference increased from 188+/-29.0mmHg to 196+/-25.0mmHg, but was not statistically significant(p>0.05). 5) Pulmonary shunting increased from 9.0+/-1.83% to 9.2+/-1.50%, but was not statistically significant.(p>0.05). The above findings suggest that pulmonary shunting in patients with normal lung function will not affected during induced hypotension with SNP.