_Methods._--The human gastric juice was obtained from Mr. V., the gastric fistula case of our laboratory. The chemical and digestive characters of his juice are discussed in a recent paper.[87] In the different experiments, different samples of gastric juice were used. The secretin employed was always freshly prepared. Digestion was carried out in the incubator at 38 C. with the reaction of 0.4 per cent. acid, and the end of the period was marked by either boiling the mixture or (in the first two experiments) by turning the mixture alkaline. The action of the preparation, we proved, was not influenced by the method used. The dogs on which the preparations were tested were prepared for carotid blood pressure, injection into the external jugular vein, and cannula in the pancreatic duct, essentially the methods of Bayliss and Starling[32] being employed. The preparations were injected at body temperature after being neutralized and filtered. Except for the addition of normal salt solution instead of gastric juice, the control injections of secretin were submitted to exactly the same treatment as the other preparations. [87] Carlson: Am. Jour. Physiol. =38=:248, 1915. _Results._--Our results are embodied in Table 1. We assured ourselves before beginning the series that incubation of secretin with _boiled_ gastric juice produced no change. It is to be noted in the table that each experiment is a unit complete in itself, beginning and ending with a control injection of secretin. _Special attention is called to the marked destruction that follows contact of human gastric juice with secretin for merely one minute._ In Experiment 4, using 1 c.c. of human gastric juice, the action fell to 14 drops from an original secretion of 21; in Experiment 5, using 8 c.c. of gastric juice, the action fell to 6 drops from an original secretion of 20. Of interest also is the rate at which we get _complete_ destruction of secretin. This is practically 2 hours for 2 c.c. with secretin giving originally 110 drops (Experiment 2, Fig. 1), or 30 minutes for 5 c.c. with a secretin giving originally 53 drops (Experiment 6). These results are practically parallel, though they were obtained with different samples of gastric juice and in different experiments. TABLE 1.--THE DESTRUCTION OF SECRETIN BY HUMAN GASTRIC JUICE