Four preruminant Friesian calves were fitted with an innervated abomasal fundic pouch cut from the lesser curvature. They were fed twice daily a milk substitute containing only spray-dried skim-milk powder as protein source. The daily outflow of gastric juice regularly increased from 400 to 1 800 g/day between 2 and 16 weeks of age and did not change later. The daily outflow/100 kg liveweight increased until 7 to 9 weeks of age and decreased later until 30 weeks (fig. 1). The pH of the secretion always ranged between 1 and 2, except during the days following surgery and when the calves were ill (fig. 2). Expressed in absolute terms, the total outflow of hydrogen, sodium, potassium and chloride ions increased with age; this was mainly due to the increase in the outflow of gastric juice since the ion concentration showed a cyclic pattern with age (fig. 3). When expressed on a liveweight basis, the daily ion output alternatively showed increasing and decreasing periods, mainly in relation to changes in concentration, but no significant trend with age was observed (fig. 4).