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In this study, we have investigated the distribution of adenosine deaminase (ADA) in embryonic, extra-embryonic, and decidual tissues of the developing mouse embryo. ADA catalyzes a key step in purine metabolism converting adenosine to inosine. ADA specific activity (nmol/min/μg protein) was present at low levels in the embryo-decidual unit during the first 2 days of postimplantation development but then increased starting late on Day 6 of gestation (Day 0 plug). By Day 9, ADA specific activity was 80-fold higher than on Day 6. A histochemical staining method for ADA activity was applied to cryostat sections of the implantation site. The developmental increase localized primarily to the trophoblast/antimesometrial decidua interface between Days 7 and 9 of gestation, and decidua basalis and the metrial gland by Day 11. Immunofluorescent staining with sheep anti-mouse ADA antiserum confirmed the presence of ADA antigenicity in tissues forming the maternal/fetal interface. ADA specific activity was 19-fold higher in homogenates of the Day 11 decidua/parietal yolk sac than in the thymus, a tissue generally thought of as ADA-rich. High levels of ADA activity and immunoreactivity were also detected in the embryonal plasma during organogenesis, but the embryo proper showed only low levels. These results indicate that ADA is tightly regulated within tissues forming the maternal/fetal interface during early postimplantation stages of development.