In 1947 Alden showed that the quantity of osmiophilic fat in the uterine epithelium of the rat followed a cyclic variation which was related to the ovarian cycle. Minimal amounts of fat were present during the estrous phase of the cycle and maximal amounts appeared in diestrus and early pregnancy. Treatment of spayed animals with estrogen and progesterone modified uterine fat, indicating that these hormones are primarily responsible for the observed changes in cycling animals. Similar findings have been reported for the mouse (Bourg, '29), guinea pig (Nicol and Snell, '55; Burgos and Wislocki, '56) cattle (Skjerven, '56) human (Gillman, '41 ; Page et al., '51) and rabbit (Gilbert, '42).
As pointed out by Alden ('47) it is unlikely that osmiophilic epithelial f a t represents the total intracellular fat of the uterus, since it is known that fat bound to proteins is not visibly colored by osmication or fat-soluble dyes. I n confirmation of this conclusion, Berg ( %), using the benzpyrenecaffeine technique for "masked" lipids, was able to demonstrate nearly as much lipid in the endometrial stroma as in the epithelium of the mouse uterus.
Experiments reported here were designed to determine whether neutral fat and phospholipids of the uterus, extracted