We have tested whether the ions K+, Na+, or Cl- are required for either initiation or maintenance of secretory parameters of amphibian oxyntic cells. In vitro mucosal preparations were washed repeatedly in Ringer solution lacking the ion in question either before (i.e., resting tissues) or after addition of stimulants. Electrophysiology, H+ secretion, and morphology (morphometric analysis of oxyntic cells) were monitored. Na+ or K+. Stimulation of resting tissues in Na+ or K+-free solutions elicited no H+ secretion; there was only a small increase (approximately twofold) in secretory surface density (Sv) (during control Sv increased sixfold). The Na+-free effects were partially reversed by high [K+]. In secreting tissues, Na+ or K+-free solutions caused H+ secretion to decrease to zero. Gastric glands from these mucosae appeared "occluded": the gland lumen was filled with apposed apical projections of oxyntic cells. Cl-. Stimulation after Cl- removal initiated low levels of H+ secretion. Morphologically the cells either appeared normally stimulated (Sv increased eightfold) or, more rarely, took on the occluded appearance. Cl- removal from stimulated mucosae decreased H+ secretion approximately 50%, but the cells were normally elaborate. We conclude that K+ is required for the initiation of H+ secretion and the stimulated morphology change; the effects of Na+ removal are due to effects on internal [K+]; Cl- is not required. The occurrence of occluded morphology under conditions of inhibited H+ and H2O secretion indicates a role for water flow in the maintenance of stimulated morphology.