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Previous studies have shown that a silicone rubber balloon filled with a hyperosmolar solution of contrast medium expands and may rupture because the wall acts as a semipermcable membrane. The deflation of latex rubber balloons in vivo has been attributed to the same cause. The authors investigated the behavior of latex rubber balloons filled with metrizamide solutions ranging from hyperosmolar to hypo-osmolar relative to plasma. When balloons were immersed in plasma, tritiated water passed bidirectionally through the wall at a rate unrelated to the concentration or volume of the contrast medium in the balloon. There was no exchange of sodium ions across the wall, and the osmolality of the metrizamide did not change over a period of up to six weeks. The authors conclude that deflation of a latex rubber balloon during the first few hours or days is not due to osmosis.