Since our experimental results suggest that UT-B1 functions as active water transporter against osmotic gradient in C6 glial cells, we report here for the first time the evidence for the active water transport. Exposure of C6 cells to a hyperosmotic solution containing glycerol or sucrose produced cell shrinkage due to water efflux according to osmotic gradient for water movement. On the other hand, C6 cells show cell swelling against osmotic gradient for water movement just after exposure to a hyperosmotic solution containing urea, indicating that water influx against osmotic gradient for water movement is accelerated by urea; i.e., urea performs active water transport. A specific inhibitor of UT-B, pCMBS, blocked the urea-induced swelling. The urea-induced cell swelling was significantly suppressed in the siRNA-induced UT-B1-knockdown C6 cells. Taken together, these observations indicate that UT-B1 acts as an active water transporter.. providing a new model on active water transport. (c) 2006 Elsevier Inc. All rights reserved.