The pesticide methoxychlor (MXC) exhibits estrogenic activity although it is not a steriod. Therefore its mode of action may differ from that of estrogen. Here the author has evaluated the ability of MXC to replace estrogen in the ovariectomized, hormone-treated decidual cell response (DCR) model. Following priming with estrone, ovariectomized rats were treated with estrone plus progesterone, progesterone alone, or progesterone plus various dosage levels of MXC. Within a narrow dose range, MXC can replace estrone and, in combination with progesterone, produce a maximal DCR. In the same manner as that seen with progesterone plus estrone, progesterone plus MXC produced no effect different from progesterone alone at low to intermediate dosages and in inhibition of decidual growth at high doses. The data support the hypothesis that MXC exhibits classical mechanisms of estrongenic activity.