T he effect of methylprednisolone (2 mg/kg per day given parenterally for 3 doses, 2 wk or 12 wk) on the permeability of mammalian gastric mucosa to hydrogen ion (H⁺) was examined with denervated fundic pouches in dogs with antrectomies. Transmucosal electric potential difference (PD) and net fluxes of H⁺ and Na⁺ were determined for luminal [H⁺] from 20 to 160 mM and [Na⁺] from 1 to 140 mM ([H⁺] and [Na⁺] were varied reciprocally). The PD was 50-60 mV lumen negative and was constant over the entire range of Na⁺ and H⁺ concentration tested. Net H⁺ flux varied linearly with [H⁺]. Extrapolation indicated apparent H⁺ loss at zero luminal concentration, suggesting a basal HCO₃⁻ secretion. Addition of acetylsalicylic acid (ASA) or taurocholate decreased the PD to 30-40 mV and increased threefold the slope of the relation between net H⁺ flux and [H⁺] (k_H). Calculation of PD-independent permeability constants for H⁺ (P_H) with the Goldman constant field equation indicated that this increase in k_H could not be attributed solely to the associated decrease in PD. Prednisolone administered for 3 doses had no effect on either the basal mucosal permeability to H⁺ or the altered permeability induced by ASA or taurocholate. Chronic administration induced a low rate of basal acid secretion (at 12 wk) but had no effect on either PD or k_H. However, the increase in k_H and P_H that developed upon addition of ASA or taurocholate in chronically treated dogs was more than one and a half times that of controls. These data suggest that prolonged treatment with glucocorticoids increases susceptibility of the gastric mucosa to damage by agents that increase permeability to H⁺.