The vasopressin (VP)-induced increase in water permeability in high-resistance, amphibian epithelia is not altered by the abolition of net Na + flux caused by amilofide added to the apical bathing medium. In this work we looked at the effects on water transport of amiloride added to the serosal medium at a concentration (10-3 M) known to inhibit Na +/H + exchange. In urinary bladders of Bufo marinus, amiloride partially blocked the hydrosmotic response to VP. A similar inhibition was found with cyclic adenosine 5'-monophosphate (cAMP) or serosat hypertonicity. We hypothesized that this effect of amiloride could be due to an inhibition of Na+/H + and/or Na+/Ca 2+ antiporters present in the epithelial basolateral membrane and looked at the effects of the diuretic in Na +-free media. A similar degree of inhibition of water flow was still found, thus showing that amiloride acts on a cell target other than the antiporters. In toad skin, amitoride did not inhibit the hydrosmotic response to VP and to isoproterenol; however the response to high K + was significantly reduced. Among the amiloride cell targets described so far, adenytate cyclase and protein kinase A appear to be the best candidates to explain the inhibition of the hydrosmotic response reported here. Direct measurements of intracellular cAMP are needed however to substantiate this hypothesis.