The interplay of maternal vascular system, maternal and fetal factors in determination of the site of implantation and placentation will be studied in the hamster. The sequence of changes in the ovum, ovary and uterus will be compared in cyclic, pseudopregnant and pregnant hamsters, in surgically or endocrinologically altered females and in those bearing IUDS. Pituitary and serum LH by RIA will be analysed. Control of decidualization in the pseudopregnant hamster, stimulus for induction of deciduomata, timing of sensitivity to induction, spacing, and life history of deciduomata will be investigated. The effect of gerontology, previous pregnancies and pseudopregnancies on the above factors will be considered and investigation of immunological aspects. Immunological studies (hamster only) will include assessment of immunological reaction at varying ages, and effect of antisera formed in the rabbit to hamster thymus or lymphocytes, deciduomata, the placenta and its components on specific stages of the physiological states. The study of the IUD is being extended to the ferret.