The post-natal cochlear mammalian epithelium have no capacity to proliferate in tissue, however, dissociated supporting cells exhibit the ability to divide and trans-differentiate into new hair cells in vitro, with this process found to be correlated with the downregulation of the cyclin-dependent kinase inhibitor p27(kip1). Here we show that knockdown of p27(kip1) with short hairpin RNA-expressing vectors results in the cell-cycle reentry of post-mitotic supporting cells in the post-natal mouse cochleae ex vivo. The p27(kip1)-knockdown cells incorporated BrdU, and then divided into two daughter cells. However, there was also activation of the apoptotic pathway in some supporting cells. These results indicate that the use of RNA interference to target p27(kip1) is an effective strategy for inducing cell-cycle reentry in post-mitotic supporting cells in the post-natal mammalian cochleae, although additional manipulations of the supporting cells are required to achieve hair cell regeneration.