1. The urinary acification system present in the bladder of the toad Bufo marinus ictericus was investigated by means of an improved technique of mounting the membrane that minimizes the edge damage when the bladder is placed as a sheet between lucite half chambers. 2. In ouabain-treated bladders in the absence of exogenous CO2, acidification rates were similar to those observed for turtle bladder. 3. The presence of 1% CO2 at the serosal gas phase increased proton secretion (JH). Stimulation of JH was also observed when the cell pH was decreased by back diffusion of salicylic acid added to the mucosal (M) compartment. 4. The estimate of the passive proton permeability of this epithelium as a whole yielded values around 1 x 10(-4) cm/s. The maximum pH gradient that could be established across the membrane in the short circuit condition (about 3 pH units) was taken as the apparent proton-motive force (PMF') of the system and these values were similar to those observed in the turtle bladder. 5. Luminal membrane depolarization caused by substitution of NaCl by KCl in Ringer solution led to an increase in JH at M pH = 7.3 without altering the PMF', which suggests that the electrical potential difference across the luminal membrane in ouabain-treated bladders is negligible when M pH is sufficiently acid to abolish H+ secretion.