Regucalcin is involved in maintenance of calcium homeostasis due to the activation of Ca2+ pumping enzymes in the plasma membrane. It has a suppressive effect in cell proliferation, DNA and RNA synthesis, and may be associated with the abnormal cell division on tumor tissues. On the other hand both estrogens and Ca2+ are implicated in breast and prostate cancer but there are no studies focused on the expression of regucalcin in rat mammary gland or prostate. Furthermore, it is known that the expression of regucalcin in rat liver and kidney is regulated by 17beta-estradiol (E2). The aim of this study is to analyze if regucalcin is expressed in rat mammary gland and prostate and if it is regulated by E2 in these tissues. We demonstrated for the first time that regucalcin mRNA and protein are present in rat mammary gland and prostate by in situ hybridization and immunohistochemistry, respectively. Furthermore, we show by Real-time PCR that E2 down-regulates regucalcin expression in rat mammary gland and prostate.