The change in pulmonary extravascular water calculated from indicator-dilution curves recorded by 125 RISA and 131 iodoantipyrene was compared to the weighed lung water in five groups of dogs. Four of these groups were kept hypotensive for 2 hr by hemorrhage, then reinfused with blood. They then received isoproteranol, methylprednisolone, phenoxybenzamine, or no drug. A fifth group was anesthetized for 4 hr, but kept normotensive. Hypotension, or prolonged anesthesia caused a recruitment of pulmonary capillaries in spite of a decreased cardiac output. This recruitment was decreased by isoproteranol or methylprednisolone and was unaffected by phenoxybenzamine. The recovery of RISA decreased during the course of all experiments. This decrease may be due to segmental areas of venous obstruction. Only in the group of animals receiving phenoxybenzamine after hypotension were the typical changes of congestive atelectasis seen.