Brain poly(A)mRNA from Fischer 344 rats at different ages throughout the lifespan will be analyzed with RNA-driven hybridization reactions with non-repetitive DNA to establish limits on the extent of transcriptional changes. The decidual response during pregnancy in aging C57BL/6J mice will be analyzed in relation to plasma estradiol and progesterone. It is possible that the previously observed major impairments of the experimentally induced decidual response represents a change in threshold for decidualization, rather than a true limitation.