The distribution of hydroxysteroid sehydrogenase (HSD) was histochemically examined in the labyrinth of the dog placentae. Except at term, weak activities of 3B-HSD were detected in the trophoblast without change in activity during pregnancy. By electron microscopy neither cytotrophoblast nor syncytiotrophoblast seemed to be a steroid-secreting cell . The results suggest thet the trophoblast cannot synthesize steroid hormones de novo in the dog placenta.
