Structural changes of the cytoplasm of urinary bladder granular cells after an antidiuretic hormone (ADH) stimulation of water transport were studied using standard and cryogenic methods of electron microscopy. Numerous changes occurred in these cells, the cytoplasm of the granular cells becoming swollen, and the intercellular spaces enlarged. Most granules become fused with the apical membrane. Under maximal ADH action, giant vacuoles appear in the cytoplasm of granular cells, in association with microfilaments and microtubules. Analysis of ultrastructure of the granular cells has established the origin of giant vacuoles from the cis -cisterna of the Golgi complex. A hypothesis based on the morphofunctional homology of giant vacuoles in granular cells with the contractile vacuoles of Protozoa is proposed in which the giant vacuoles (‘contractile-like’ vacuoles) are seen as operating a osmoregulatory role in these cells. It is also proposed that microtubules and microfilaments participate in giant vacuole migration through the cytoplasm.