Multidrug resistance phosphoglycoprotein (ABCB1) has been shown to limit maternal-fetal transfer by actively excluding ABCB1 substrates. The authors have previously demonstrated a marked decrease in placental ABCB1 expression in the human and mouse with advancing gestation. In the present study, it is hypothesized that the decrease in ABCB1 expression will result in increased transplacental transfer of ABCB1 substrates over the second half of gestation and that progesterone exhibits a regulatory role on placental ABCB1 expression and function. The authors demonstrate a significant increase in transplacental transfer of [(3)H]digoxin (an ABCB1 substrate) in late gestation (E18.5; P < .001) when compared to earlier embryonic days. Furthermore, maternal plasma progesterone levels did not influence expression or function of ABCB1. The authors conclude that the fetus is increasingly exposed to both endogenous and exogenous substrates of ABCB1 present in the maternal circulation with advancing gestation and that progesterone does not elicit a regulatory role on placental ABCB1 expression or function in vivo.