The aim of this study was to elucidate the expression pattern of ABCG2 in the placenta from the mid stage to the end of gestation. rABCG2 expression was investigated in rats on the 14th gestation day (gd) and the 20th gd. Expression of the rABCG2 gene and expression of rABCG2 protein in the placenta were detected on gd 14 by RT-PCR and Western blot analysis respectively. The expression level of rABCG2 on gd 20 was less than that on gd 14. We investigated whether progesterone, secreted from the placenta, regulates the expression of ABCG2 in BeWo cells. Expression levels of the ABCG2 gene and protein in BeWo cells were decreased by progesterone treatment. We conclude that progesterone plays a role in reduction in the expression level of ABCG2 in the placenta with the advance of gestation from the mid stage to the end of gestation.