We have previously demonstrated that in renal cortical collecting duct cells (RCCD1) the expression of the water channel Aquaporin 2 (AQP2) raises the rate of cell proliferation. In this study, we investigated the mechanisms involved in this process, focusing on the putative link between AQP2 expression, cell volume changes, and regulatory volume decrease activity (RVD). Two renal cell lines were used: WT-RCCD1 (not expressing aquaporins) and AQP2-RCCD1 (transfected with AQP2). Our results showed that when most RCCD1 cells are in the G1-phase (unsynchronized), the blockage of barium-sensitive K+ channels implicated in rapid RVD inhibits cell proliferation only in AQP2-RCCD1 cells. Though cells in the S-phase (synchronized) had a remarkable increase in size, this enhancement was higher and was accompanied by a significant down-regulation in the rapid RVD response only in AQP2-RCCD1 cells. This decrease in the RVD activity did not correlate with changes in AQP2 function or expression, demonstrating that AQP2—besides increasing water permeability—would play some other role. These observations together with evidence implying a cell-sizing mechanism that shortens the cell cycle of large cells, let us to propose that during nutrient uptake, in early G1, volume tends to increase but it may be efficiently regulated by an AQP2-dependent mechanism, inducing the rapid activation of RVD channels. This mechanism would be down-regulated when volume needs to be increased in order to proceed into the S-phase. Therefore, during cell cycle, a coordinated modulation of the RVD activity may contribute to accelerate proliferation of cells expressing AQP2. J. Cell. Biochem. 113: 3721–3729, 2012. © 2012 Wiley Periodicals, Inc.