The natriferic and hydroosmotic responses to vasopressin (ADH) of the isolated toad bladder were studied as a function of the serosal bath NaCl concentration. Bladders were preincubated in Ringer's solution containing equiosmotic substitutions of mannitol, choline chloride, or choline sulfate for NaCl. Their hydroosmotic and natriferic responses to ADH were measured in NaCl Ringer's solution. Bladders previously bathed on their serosal surface with mannitol had a markedly diminished hydroosmotic response, whereas the natriferic response of these bladders was essentially normal. Similarly, preincubation with either choline chloride or sulfate also inhibited the hydroosmotic response, although the decline was substantially less than that with mannitol. Preincubation with mannitol also inhibited the hydroosmotic response to dibutyryl cAMP. Therefore, the isosmotic substitution of serosal bath NaCl with mannitol decreases primarily the hydroosmotic response to ADH at a post-AMP step. The natriferic effect of the hormone remains essentially intact under these conditions. Different compartments (or cell types) appear to regulate the action of ADH on sodium and on water transport in this tissue because of the selective effects of preincubation with low NaCl Ringer's solution on the hydroosmotic and natriferic responses to ADH.