Calbindin-D(9k) (CaBP-9k) is a cytosolic calcium binding protein mainly expressed in duodenum, placenta and uterus. In order to understand the expression pattern and regulation of uterine CaBP-9k gene, the expression of CaBP-9k mRNA and its regulation by estrogen (E2) and progesterone (P4) were investigated in the mouse uterus during late pregnancy (from day 12 to 18) and lactation. The expression levels of uterine CaBP-9k, estrogen receptor alpha (ERalpha) and progesterone receptor (PR) mRNAs were measured by Northern blot analysis. The expression levels of mouse uterine CaBP-9k mRNA gradually increased from pregnancy day 16 (P16), peaked at P18 (6.0-fold vs. P12) and declined at birth and during lactation. The expression levels of ERalpha and PR mRNAs indicated a similar fluctuation as CaBP-9k mRNA, suggesting the role of sex steroids/receptors in the regulation of CaBP-9k gene. To investigate effect of steroid hormone on CaBP-9k mRNA expression, three groups of animals were injected (s.c) with steroid hormone antagonists (RU486, tamoxifen, and ICI182780), respectively. RU486, a P4 antagonist, induced a significant decrease in CaBP-9k mRNA expression at 48 (3.2-fold) and 72 h (3.8-fold). However, tamoxifen and ICI182780, E2 antagonists, had no effect on CaBP-9k mRNA expression. Combined treatment with RU486 and ICI182780 did not further decrease the expression level of CaBP-9k mRNA when compared with RU486 treatment at 48 and 72 h. In addition, the treatment with RU40555, a glucocorticoid/progesterone antagonist, resulted in a decrease at 48 and 72 h following treatment. These results indicate that E2 is not likely involved in the regulation of CaBP-9k gene in the mouse uterus during late pregnancy and lactation. In conclusion, the present results suggest that P4, not E2 is a key regulator of CaBP-9k mRNA expression during late pregnancy and lactation.