Continuous measurements of intraduodenal pH changes were made in five healthy conscious dogs by means of a miniature pH electrode inserted into the duodenum through a chronically implanted duodenal cannula. Gastric motor activity was also recorded simultaneously by means of a chronically implanted force transducer. When dogs were fed once a day with a measured volume of dog food, it was found that 24-h pH changes in the duodenal contents could be divided into three major periods: the weak acid period, the strong acid period, and alkaline period. The weak acid period lasted for 9.2 +/- 0.43 h after feeding with the duodenal pH ranging from 7.0 to 4.0. The weak acid period was gradually changed to the strong acid period in which the duodenal pH fluctuated widely from 8.0 to 1.0. This period lasted for 5.2 +/- 0.26 h. After the strong acid period, pH of the duodenum abruptly turned alkaline and remained between 7.0 and 8.3 until the next meal. These three periods of pH change are closely related to those of three different gastric motor patterns; the digestive, intermediate, and interdigestive patterns, respectively.