1. The electrical activity of the antrum and duodenum was recorded from chronically implanted electrodes in six conscious dogs fitted with gastric and/or duodenal cannulae. Antral and duodenal pH were concomitantly registered using glass electrodes inserted through the respective cannula. 2. In dogs fasted for 18-24 h an alkalinization of the antrum of about 5 pH units lasting 15-30 min occurred regularly at 90-120 min intervals at the cessation of a period of antral spiking activity. This antral alkaline period was accompanied by the reflex of intestinal juice into the stomach, as determined by the presence of bile salts in gastric samples. 3. The antral pH rise was related to the development of a phase of regular spiking activity (RSA), or activity front, on the proximal duodenum concomitant with the motor quiescence of the antrum. The rise in pH disappeared after occlusion of the pylorus, which prevented the passage of duodenal contents into the antrum. 4. It is concluded that the mean antral pH value is 5.8 for half the recording time in fasted dogs, the causative factor of this alkalinization being the regurgitation of duodenal contents into the antrum while quiescent. Accordingly, more alkaline duodenal contents were observed during the occurrence of the RSA phase and the following gastro-duodenal quiescence; then the progressive acidification of the gastric and duodenal contents when the motility reappeared may be an inducing factor of a new RSA phase.