The water permeability of ADH target epithelial cells is believed to be regulated by a cycle of exo-endocytosis of vesicles containing functional water channels. These vesicles were selectively labeled in intact frog urinary bladders with an impermeant fluorescent marker, 6-carboxyfluorescein. Vesicle suspensions containing the labeled endosomes were obtained by homogenization and differential centrifugation of bladder epithelial cells. The osmotic permeability of the endocytic vesicles was measured, using a stopped-flow fluorescence technique, in the absence or in the presence of HgCl2. This permeability was found very high (500 microns/sec) and inhibited by 1 mM HgCl2 (90%), thus confirming the presence of water channels. The labeled endosomes were then separated from the other membrane vesicles by flow cytometry and sorting. Their protein content was analyzed by electrophoresis on ultrathin polyacrylamide gels. Two double bands were found at 71 and 55 kDa as well as a small band at 43 kDa. They respectively correspond to 31, 38 and 10% of the total amount of silver-stained proteins present in the sorted endosomes, while they only represent 2, 4, and less than 1% of the proteins contained in the vesicle suspension, before sorting. These highly enriched proteins (or at least one of them) are likely to be involved in the mechanism of water transport. Associated to their partial purification by differential centrifugation, the sorting of the endosomes by flow cytometry seems a good way to further characterize the water channel.