A blood-gas study of the respiratory effects of sodium nitroprusside during anaesthesia has shown a marked reduction in PaOi when the drug was administered. After nitroprusside PaO2 returned to the previous values. It is suggested that the reduction in PaO2 is a result of an increased scatter of ventilation/perfusion relationships in the lung. The reduction in PaO2 was evident during both spontaneous and artificial ventilation. Nitroprusside was associated with a small decrease in actual bicarbonate.