1. The effects on gastric secretion of altering the amount and type of food eaten were examined in sheep with fundic abomasal pouches and in sheep which also had the antral region of the abomasum isolated into a pouch or removed (antrectomy). Secretion, which was continuous in all preparations, was collected over 24 h periods, and daily determinations of its acid and pepsin concentrations were made for periods of up to 10 weeks.
2. Experimental diets consisting of chaffed lucerne, meadow and wheaten hays were fed on ad lib. or restricted regimens.
3. Raising and lowering the dry matter (DM) intakes of lucerne chaff increased and decreased respectively the volume, acidity and acid and pepsin outputs (volume x concentration of acid or pepsin) of fundic pouch secretion.
4. Increases of 27–64% in the amount of lucerne chaff eaten, after changing from restricted to ad lib. feeding, were followed by increases in the volume (19–66%), acid concentration (4–10 mequiv. H+/l, 3–9%) and acid output (18–76%) of pouch secretion.
5. With DM intakes of mixed lucerne and wheaten chaffs between 88 and 107% of those of lucerne, the secretion from the pouches was reduced to 45–88% of the volume and 39–77% of the acid output observed with the lucerne diet. Acid concentration was least affected, being unchanged in one series of observations and decreased by, at the most, 13 mequiv. H+/l (12%) in another. Reverting to a diet of lucerne chaff reversed these effects: the volume was increased by 30–49%, acid concentration by 2–15 mequiv. H+/l (2–14%), acid output by 38–68% and pepsin output by 30–43% although the intake of DM was the same or 6% less than that on the wheaten chaff mixture.
6. Secretion was greater when animals ate lucerne chaff than when they ate meadow chaff.
7. The changes in secretion according to diet were obtained in animals with antral pouches and antrectomy as well as in those with only fundic pouches.
8. It is argued in discussion that the markedly different abomasal secretions on different diets arise from abomasal stimulation due to the nature rather than the amount of digesta entering it and that although the pyloric antrum contributes to these changes it is not essential for their occurrence.