The effect of sodium nitroprusside(SNP) on arterial oxygen tension in 20 neurosurgical patients with normal lung function was studied under general anesthesia. Blood gas, heart rate, mean arterial pressure and central venous pressure were measured before, during and after SNP infusion. The results were as follows:1) Arterial oxygen tension significantly decreased from 177±40.6 mmHg before SNP to 138±50.1 mmHg during SNP(p<0.005), and increased again to 168±44.4 mmHg after SNP. 2) Mixed venous oxygen tension decreased 47±8.2 mmHg to 40±7.3 mmHg(p<0.005), and increased again to 44±10.3 mmHg. 3) Heart rate significantly increased from 92±24.6 beats/min to 118±27.4 beats/min(p<0.005), and decreased again to 94±18.6 beats/min. The above findings have shown a marked reduction in PaO2 when SNP was administered during general anesthesia. After SNP, PaO2, returned to the previous values. It is suggested that the reduction in PaO2, is the result of an increased scatter of ventilation/perfusion relationships in the lung and inhibition of hypoxic pulmonary vasoconstriction by SNP. These results provide evidence that SNP induced hypotension may cause significant impairment in pulmonary gas exchange in patients with normal lung function.