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We evaluated tubular handling of urographic contrast media in normal and obstructed states by the use of a model which allowed direct evaluation of transcellular flux. Segments of rabbit renal tubule were harvested by microdissection. Individual tubular segments were cannulated by micropipette while immersed in a serum bath. Radioactive iodine-labeled sodium iothalamate was added to the perfusate of normal and distally obstructed nephron segments. Any iothalamate that moved through the tubule wall was recovered in the bath. The tubular epithelium was counted to determine epithelial concentration of iothalamate. A small efflux of contrast medium occurred which was unaffected by acute obstruction and increased pressure. Proximal straight tubules were more permeable to sodium iothalamate than cortical collecting tubules.