2-Mercapto-1-(beta-4-pyridethyl) benzimidazole (MPB) was originally introduced as a reversible inhibitor of RNA synthesis, but subsequent findings made this suggestion doubtful. We examined the effect of MPB on active sodium transport, measured as short-circuit current (scc), across the isolated urinary bladder of the toad (Bufo marinus). The drug caused a rapid, dose-dependent inhibition of baseline scc; 25 micrograms/ml MPB reduced it by 70%. Sensitivity to MPB was the same in the presence and absence of metabolizable substrate. The transport stimulation by aldosterone (7 X 10(-8)M) was abolished entirely when MPB was introduced 30 min before the hormone. In bladders incubated with MPB with or without aldosterone, removal of both agents resulted in a rise in scc, which was more rapid in the aldosterone-pretreated hemibladders; a significant difference was observed after 30 min. This suggests that MPB inhibited transport at a site distal to messenger RNA accumulation. The effect of 3 hr of pretreatment with MPB on the response of the bladders to antidiuretic hormone (ADH, 20 mU) and cyclic AMP (cAMP, 10 mM) was then examined. The absolute increment in scc due to these agents was the same as in the absence of MPB, though the baseline was much reduced by the drug. After challenging MPB-pretreated bladders with theophylline (22.5 mM), sodium transport rose continuously for 90 min, in contrast to the small, short-lived rise in the absence of MPB. It is proposed that, in the toad bladder, MPB may: (1) inhibit cAMP-dependent protein kinase, as found by us in other tissues; and (2) counteract the accumulation of a transport inhibitor, possibly calcium or cyclic GMP, in tissues treated with endogenous or exogenous cAMP.