The role of the mineralocorticoid receptor (MR) in salt and water balance is well established. What remains less clear is its role in non-classical tissues. Our interest in the role of the MR in ovarian biology, led us to establish a granulosa cell specific MR-null mice line. We have previously described the abnormal phenotype of the KO ovaries observed in these mice: diffuse stroma, degenerate oocytes and vacuolated/disorganised follicles, theca and corpora lutea. The aim of these studies was to investigate the expression of proteins and genes by the ovary which may be regulated by the MR. The granulosa cell specific MR-null mouse was generated by crossing AMHR2cre mice (provided by Prof M. Matzuk, BCM) with our MR floxed mice. Ovaries were collected from null mice and littermate controls at 7-10 weeks of age for immunohistochemical and gene expression analyses. Formalin-fixed ovarian sections were immunostained for the MR, progesterone receptor (PR) and 11β-hydroxy steroid dehydrogenase 2 (11β-HSD2). RNA was extracted from whole ovaries and prepared for gene expression analysis using the Fluidigm Biomark™ HD system with Taqman primers. MR immunohistochemistry was used primarily to confirm genotyping results; these analyses did not always correspond, with some null ovaries having detectable protein in their granulosa cells (GC). Both the PR and 11βHSD2 proteins were localised to GC and some stromal cells in control and null ovaries; only the intensity of the 11βHSD2 protein appeared reduced. Our preliminary Fluidigm analysis assessed the expression of Cox2, Wnt4, Rank-ligand, Id4, SCC, ERα, Sgk1, TGFβ1 and the PRL receptor, along with appropriate housekeeping genes. Sgk1, Rank-ligand and TGFβ1levels were increased in null mouse ovaries relative to controls whilst the other genes were unchanged. These results further support our hypothesis that the MR plays a central role in folliculogenesis.