Both pressure and oxygen content of blood in the left pulmonary artery distal to an inflated balloon simulate that observed in the wedged cardiac catheter position. Clamping the ipsilateral pulmonary veins after inflation of the left pulmonary artery balloon, in this or in the reverse order, produces a rise in pressure distal to the balloon that exceeds the resting left pulmonary artery pressure. No corresponding increase in oxygen content of blood sampled distal to the inflated balloon, compared with that in the unoccluded pulmonary artery, could be demonstrated. This contrasts with the flow of arterialised blood that had been sampled after some months of permanent left pulmonary artery occlusion from the same position. The pressures both proximal and distal to the site of permanent occlusion are approximately the same.