In the preset report, we studied if blockers of the K+/Cl- cotransporter, and Cl- and K+ channels modify the proliferation of MC3T3-E1 osteoblastic cells. A blocker of the K+/Cl- cotransporter, DIOA (100 and 500 microM), diminished the cell proliferation. NPPB (100 and 300 microM, a blocker of Cl- channel) and quinine (100 and 500 microM, a blocker of K+ channel) also suppressed the proliferation of MC3T3-E1 cells. These blockers of the K+/Cl- cotransporter/channels (DIOA, NPPB and quinine) would increase the intracellular Cl- concentration ([Cl-]i). On the other hand, our previous study (Biochem. Biophys. Res. Commun. 361, 1038-1043, 2007) reported that a decrease in [Cl-]i diminished the proliferation of MC3T3-E1 osteoblastic cells. Taken together, the observations indicate that both increases and decreases in [Cl-]i diminish the proliferation of MC3T3-E1 osteoblastic cells.