The mechanical effect of balloon distension of the duodenum on the stomach was studied in 10 mongrel dogs with a mean weight of 14.8 +/- 3.2 kg. The response of the pyloric sphincter and antrum as well as of the corpus of the stomach to duodenal distension by a balloon filled with water in increments of 2 ml, up to 6 ml, was determined. The test was repeated after anesthetizing the pyloric sphincter and antrum and the duodenum, each at a separate time. In 5 of 10 dogs the effect of duodenal distension on the vagotomized stomach was studied. Duodenal distension with 2 ml of water produced an increase in the pyloric sphincter pressure (p < 0.05) and a decrease in the antral pressure (p < 0.05); it had no effect on corporeal pressure (p > 0.05). Distension with 4 ml and 6 ml produced the same effect as 2 ml (p > 0.05). The anesthetized pyloric sphincter and antrum did not respond to duodenal distension. Likewise, the pyloric sphincter and antrum showed no response to distension of the anesthetized duodenum or of the duodenum after vagotomy. Pyloric sphincter contraction and antral dilatation upon duodenal distension suggest a reflex relation we call the duodenopyloric reflex. This reflex appears to prevent duodenopyloric reflux. Moreover, the antrum dilates probably to accommodate more gastric contents.