Segments of the gastrointestinal tract removed from rats after intervals of time following injection of S35-sulfate were fixed in aqueous formalin and then washed in water. Contact and coated autoradiograms were prepared. The suggestion made by others that more of the labelled sulfate is fixed by the mucosa than by the underlying coats of the gastrointestinal tract is confirmed. In addition it was found that the isotope is fixed to a greater extent in the lower intestine than in the middle or upper portions of it.

Coated autoradiograms revealed that 6 hours after administration of S35-sulfate more of the label was present in the goblet cells lying deep in the crypts of the mucosa than in those adjacent to the intestinal lumen. By the 24th hour the concentration of the isotope was strikingly higher and more uniform from cell to cell. The mucus in the intestinal lumen was also highly radioactive. At the end of 48 hours very little of the sulfur-35 remained in the intestinal wall or could be made out in the mucus of the lumen: the autoradiographic reaction was faint and diffuse as contrasted with the punctiform and intense reaction given by the specimens removed at the end of shorter intervals of time.

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