An earlier report indicated that acid secretion in turtle urinary bladder is driven by an unusual vacuolar H+-ATPase and that the ATPase accounts for essentially all acid secreted. These results, however, are difficult to reconcile with the acid transporters currently ascribed to the renal collecting duct. Here, we re-examine the effect of bafilomycin A1, an inhibitor of vacuolar (V-type) H+-ATPases, on acid secretion by intact isolated bladders from Pseudemys scripta turtles. Serosal-side bafilomycin had no effect on the transepithelial acidification current (AC). In the mucosal solution, bafilomycin inhibited the AC, with inhibition developing over the range 0.1-10 nmoll(-1), with a sigmoidal dose-response curve, and an IC50 of 0.47 nmoll(-1). At saturation, approximately 70% of H+ secretion was inhibited. The remaining 30% could be abolished by 30 micromoll(-1) Sch-28080, which is a level that in other systems is known to inhibit H+/K+-ATPase transport activity specifically and essentially completely. When the order of addition was reversed (Sch-28080 first), there was no change in the magnitude of the effect produced by either inhibitor, and the two together again eliminated the AC. The data indicate that baseline acid secretion in intact bladders is due (i) in part to a highly bafilomycin-sensitive process, with sensitivity typical of vacuolar H+ ATPases; and (ii) in part to a more bafilomycin-resistant process that is sensitive to Sch-28080.