We have previously reported that during the interdigestive state repeated episodes of high-amplitude contractions interrupted by long-lasting motor quiescence were seen to occur regularly until the next meal in the dog. However, none of the dogs used in our laboratory showed such regular changes in gastric motor pattern all the time during the interdigestive state. In the present study, we measured plasma motilin concentrations and intraduodenal pH together with gastric motor activity in conscious dogs. It was found that as long as intraduodenal pH remained between 7.0 and 8.5 the regular occurrence of the interdigestive contractions was closely associated with increases in the plasma immunoreactive motilin (IRM) concentration. However, when intraduodenal pH became acid, there were no typical interdigestive contractions even though plasma IRM concentration was elevated. We consider that high-amplitude contractions like the interdigestive contractions could not be evoked in the acid-secreting stomach even if the plasma IRM concentration was elevated during the interdigestive state. Acid secretion during the fasted state, the cause of which is not known, seems to be one of the main factors in disturbing the regularity of the interdigestive motor pattern.