We have been treating female white mice with massive doses of estrone (200 I.U.) painted on the skin. To our astonishment a group of 25 untreated spayed mice, whose vaginal smears were all negative, all went into estrus. It had been our custom in treating or examining animals to take them from their cage, treat, and then place in a temporary box until all animals in a cage were removed, then return them to their cage. No transmission of estrogenic substances has been noted among a large number of female mice treated with subcutaneous injections of 1–10 I.U. of estrone. To trace out the source of estrogenic stimulation we placed a standard mouse in the cage of each group receiving estrone, and one in the temporary box during a morning when the mice were being treated with estrone. A standard mouse is a young spayed adult known to respond to 2 I.U. of estrone administered subcutaneously. All of these standard mice went into estrus and remained in estrus for about 2 weeks. There could be only 2 probable routes of transmission and of reception of the estrogenic substance, i. e., from the skin, and from the excreta, and through the skin, and through the mouth. To make a crude estimate of these factors we placed a standard mouse with a muzzle on in the cage with estrone-treated animals. A mouse in, a paraffin sack, with only its head exposed, was also placed in a cage with estrone-treated animals. Two other muzzled and sacked animals were placed in the bottoms of cages of estrone-treated animals. The cages have false bottoms. Except for the period of exposure each standard mouse was caged separately.