Two different aminoacylnaphthylamidase reactions in the mouse uterus are described, a particulate one in the glandular epithelium and a vacuolar one in the luminal epithelium and at the implantation sites. Maximum epithelial enzyme activity was observed on Day 5 of pregnancy or after 6 days' progesterone treatment ofthe spayed mouse. No further increase was achieved after oestradiol superimposed on progesterone treatment. At implantation sites on Day 8, the activity was mainly localized

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... metrially to the embryo. It is suggested that the uterine epithelial activity contributes to the breakdown of uterine secretion before the attachment reaction.