The mucosal surface of toad urinary bladder was examined with scanning electron microscopy following its exposure to 20 mU/ml of vasopressin (VP), 10(-4) M 8-bromo-cAMP, 1 mM acetylcholine chloride, serosal hypertonicity, or a hypotonic bathing medium. After a 30-min exposure to VP, the arborizing ridge-like surface pattern typical of granular cells was transformed into microvilli, a response that was not dependent on transepithelial osmotic water movement. An identical response occurred following a 30-min exposure of the bladder to 8-bromo-cAMP, again in the absence of an osmotic gradient. Microvillus formation was not observed when cell volume was increased by incubation of tissue in half-normal amphibian Ringer's solution for 30 min, or with exposure to acetylcholine, which caused accentuation of the convexity of the apical surface of the granular cell similar to that observed with VP-induced osmotic water flow. However, 60 min of incubation in a hypertonic serosal medium (mannitol, 240 mM) caused transformation of ridges to microvilli mimicking the picture obtained with VP. These findings establish that transepithelial osmotic water flow with cell swelling is not required for microvillus formation on the apical surface of granular cells following VP stimulation, and that the surface changes are not due to cell swelling alone or to changes in the configuration of the apical plasmalemma. The results also suggest that the response to VP is mediated via the generation of cAMP. Finally, this study demonstrates that serosal hypertonicity also causes transformation of ridges to microvilli by a mechanism that is yet to be defined.