The effects of changes in nutrient concentrations of K+, Cl- and Na+ on the transmucosal potential difference (PD) and the resistance were compared for secreting fundus and resting fundus of Rana pipiens. Increase of K+ from 4 to 40 mM, decrease of Cl- from 81 to 8.1 mM and decrease of Na+ from 102 to 10 mM gave, 10 min after the change in the secreting fundus, delta PD values of -28.2, -19.8 and -7.5 mV, respectively. In the resting fundus with SCN- inhibition, the same changes in nutrient ion concentration gave delta PD values of -20.1, -17.0 and -10.2 mV, respectively. Changes in Na+ concentration were considered in a set of experiments of high acid secreting stomachs (4 to 6 mu equiv. . h-1 . cm-2). Here, delta PD gave for 10-fold decreases in Na+ concentration in secreting fundus -4.8 mV and in resting fundus with SCN- inhibition -22.6 mV. Omeprazole inhibition gave results quite similar to those with SCN- inhibition. From these results in going from secretion to inhibition, it follows that the increment of K+ conductance if it increased was lower than the increase in NaCl symport conductance since the change in delta PD for K+ decreased and that for Na+ increased. Also HCO3- conductance increased with inhibition. After SCN- inhibition the transmucosal resistance initially increased and later decreased. The decrease can be accounted for by the increase in conductance of the NaCl symport pathway and of the HCO3- pathway.