Pulmonary shunling (&OV0422;t,/&OV0422;t, with FIO2= 1) was measured in 18 anesthetized patients during deliberate hypotension. Hypotension was induced in 12 patients with sodium nitroprusside and light halothane anesthesia and in six others with deep halothane anesthesia and mechanical hyperventilation. Similar results were observed in the two groups. During the hypotensive period mean arterial pressure (MAP) was reduced to 49 ± 2 torr, a 37 per cent decrease from the control level after the onset of operation and a 40 per cent decrease compared with the recovery level during closure of the wound. &OV0422;t/&OV0422;t, however, remained unchanged throughout the study; 52 ± 0.9 per cent initially, 5.4 ± 0.8 per cent during hypotension, and 4.7 ± 0.5 per cent during recovery. It is concluded that pulmonary shunting need not develop during dliberate hypotension induced with either technique.