In nitrate solutions, Ag+ added to the luminal side had marked effects on transmucosal conductance and potential difference (PD). Conductance increased quickly (85% within 60 s, 420% by 10 min); PD increased initially (11% within 30 s) and then fell precipitously (58% decreased within 2 min, 85% decreased by 10 min). During this period, no increase in mannitol permeability was found. These changes were essentially similar in histamine-stimulated, spontaneously secreting, and metiamide-inhibited fundic mucosae. Replacement of luminal Na+ by choline had no effect on the changes observed. Similar changes occurred also in the antrum. In SO2-4 media the increases in conductance occurred more slowly (40% within 2 min, 150% after 10 min); PD increased initially for 4-6 min and then slowly declined over 60 min to 74% of control values. After Ag+ treatment, replacement of luminal SO2-4 by nitrate led to an inversion of the PD by up to 20 mV (serosa -ve). Brief exposure of the mucosa (2 min) to Ag+ did not show any obvious damage, although surface cells were damaged following more prolonged exposures. After a 4-min treatment with Ag+, electrical and secretory parameters showed substantial recoveries. Ag+ appears to increase anion conductance; these effects appear to occur on the apical membranes of tubular and/or surface cells of the fundus and antrum.