1. In conscious dogs with Heidenhain fundic pouches and denervated antral pouches control dose-response curves to gastrin pentapeptide or methacholine were constructed, both when the antral pouch was empty and when it was stimulated with ACh (0.5 g/100 ml.) in citrate buffer adjusted to pH 5.5 or 1.5.2. When the antral pouches were irrigated with ACh (0.5 g/100 ml.) at pH 1.5 the acid outputs from the Heidenhain pouch in response to graded doses of gastrin pentapeptide or methacholine were significantly less at each dose level than those obtained with ACh at pH 5.5.3. The acid outputs from the Heidenhain pouch in response to either pentapeptide or methacholine with or without antral irrigation with ACh at pH 1.5 were approximately equal. Therefore the difference between the dose-response curves obtained by stimulation with ACh at pH 5.5 and those obtained by stimulation with ACh at pH 1.5 represents gastrin release rather than release of an inhibitory hormone.4. The pepsin outputs from the Heidenhain pouch during antral irrigation with ACh at pH 1.5 were significantly less than those obtained with ACh at pH 5.5 at each dose of methacholine.5. The maximal pepsin output was significantly greater in response to methacholine than to gastrin pentapeptide.6. Gastrin pentapeptide was not a significant stimulant of pepsin secretion from the Heidenhain pouch.7. The intestinal phase of parietal secretion was not inhibited by bathing the separated antrum with acid.