SUMMARY

Intravascular or intraperitoneal injection of histamine or of the histamine releaser, '48/80', into the pseudopregnant rat produced a decidual cell reaction (DCR) in rat uteri with which there was no physical or surgical contact. The data add evidence to the role of histamine in the induction of the DCR. The histamine-releasing activity of an antihistamine (pyrathiazine) was also used to induce the DCR systemically. The appearance of the decidual cell reaction (DCR), which forms the maternal placenta from endometrial tissue, marks the morphological beginning of nidation of the fertilized egg in the rat [Krehbiel, 1937; Svetlov & Korsakova, 1955]. The epithelium is still intact at this time [Alden, 1948; Blandau, 1949], and there is no physical contact between the stroma and the trophoblast. This suggests that some metabolite of the blastocyst is being 'secreted', which diffuses through the epithelium into the endometrium, here causing DCR. In order to dissociate the DCR induction from extraneous blastocyst activities, Shelesnyak [1957] has adopted the artificial DCR (deciduoma) as a model system. In this system, the problem is the elucidation of the single factor common to all the various types of stimuli which cause DCR induction in a pseudopregnant animal. The use of the deciduoma (artificially induced DCR) offers technical advantages over the localized natural one induced by an implanting blastocyst. The mechanism of induction of the DCR is presumed to be identical in both systems. Strong presumptive evidence of the role of histamine as the DCR inducer has already been obtained. Locally (intraluminally) introduced histamine in small quan¬ tities ( 1 µg) will induce DCR; and antihistamines (independent of any of their side effects) blocks the DCR when applied to the uterine epithelium [Shelesnyak, 1952, 1957]. Support for the validity of the concept of histamine participation in DCR induction would be greatly strengthened if it could be shown that systemically administered histamine can provoke deciduomata in the hormone-sensitized uterus. The histamin