The rat calbindin (molecular mass 9 kDa) gene sequence revealed the presence of an imperfect palindromic estrogen-responsive-like element (ERE) located at position +51 from the transcriptional initiation site. This element has the sequence AGGTCAGGGTGATCT which differs by one nucleotide from the vitellogenin ERE palindromic sequence AGGTCACTGTGACCT. The activity of this element to induce transcription in response to estrogen and the ability to bind estrogen receptor was investigated. This sequence confers estrogen-dependent transcriptional activity when cloned upstream from the tkp-CAT construct in pBLCAT2 plasmid and transfected into T47D cells, similar to the vitellogenin gene ERE activity but to a lesser extent. This element binds to the estrogen receptor in vitro as assessed by gel retardation assay similar to the vitellogenin gene ERE. No such binding was detected when a mutant sequence AGATCACTGTGATCT was used. The specificity of the complex was confirmed using polyclonal antibodies, ER712 raised against the estrogen receptor. Furthermore, competition assays showed that both sequences were able to compete for binding to the estrogen receptor. The in vivo transcriptional regulation of the calbindin D-9K gene by estrogen was also investigated in the rat. Female animals maintained on a vitamin D-sufficient or a vitamin D-deficient diet were ovariectomized, housed for 3 weeks, and then injected with 17 beta-estradiol (0.5 micrograms/kg body weight/day). Slot blot analysis of total RNA showed a marked increased in calbindin D-9K mRNA levels in the uterus but not in the intestine by 26 and 50 h post-injection. These results demonstrate that the calbindin D-9K gene is transcriptionally regulated by estrogen in the uterus mediated by an estrogen-responsive element identified in the gene.