In conscious dogs with gastric fistulae and Heidenhain pouches, isoprenaline (0.75 microgram kg-1 h-1) brought about an increase in methacholine stimulated gastric acid and pepsin secretion from the innervated mucosa. 2. The increase in acid consisted for the most part of the replacement of the usual post-peak fade by a sustained plateau and of the replacement of the usual wait for the spontaneous peak by immediate stimulation of secretion.