plasma progesterone (P) and the numbers of viable embryos and resorptions were examined in aging C57BL/6J mice to elucidate possible mechanisms contributing to the loss of fertility during aging. Mice of previously proven fertility were killed on each day of pregnancy. In mice aged 3-7 months and 11-12 months, >90% mated successfully. The older mice had 12% fewer implantation sites on Days 6-10: 8.5 ± 0.06 (young); 7.5 ± 0.26 (older); (mean ± SEM), P