The simultaneous unidirectional flow of sodium into and out of isolated loops of jejunum, ileum and colon of the dog, has been measured, in vivo. Four loops of bowel observed over a 5-year period did not degenerate physiologically or histologically. The variability of sodium flux, measured repeatedly over the five years, was no greater in experiments conducted 1 day apart than in those conducted over a period of years. Adrenalectomy did not reduce this inherent variability. Moderate salt loading by intravenous administration, or by dietary intake, did not alter the transfer rates of sodium across the intestine. Approximately 1 ml of water was transferred into and out of a 20-cm length of intestine/min. Sodium concentration was usually maintained at 144 mEq/l. in the lumen of the small intestine, while sodium concentration in the lumen of the large intestine usually decreased to be partially replaced by potassium. It is suggested that the rates of sodium flow across the dog intestine, in vivo, are of the same order of magnitude per unit surface area as those of sodium flux across amphibian tissues, in vitro. While water may enter the amphibian tissue 50 times as fast as sodium, water and sodium traverse the dog intestine in the ratio of their free diffusion coefficients, water moving twice as fast as sodium.

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Potassium moves across the intestinal mucosa from the lumenal surface to the blood simultaneously with a stream in the opposite direction from blood to lumen. The net result of these two flows determines the amount of potassium appearing in the intestinal lumen. These two flows were measured across isolated loops of intestine in the dog with the use of radiopotassium. For a 20-cm length of intestine, potassium enters the lumen at 0.8–6.5 µEq/min., and simultaneously leaves the lumen at 0.7–5.2 µEq/min.