TWENTY-FOUR FIGURESDuring the last few years we have investigated the occurrence and distribution of a number of substances in mammalian placentas, using a variety of cytochemical procedures. Previous papers from this laboratory have dealt with various aspects of the human placenta (Dempsey and Wislocki, '44, '45; Wislocki and Dempsey, '45, '46a) as well as with the placentas of cats (IVislocki and Dempsey, '46b), pigs (Wislocki and Dempsey, '46c) and rodents (Wislocki, Deane and Dempsey, '46). The procedures f o r demonstrating histochemically the distribution of acid and alkaline glycerophosphatases (Comori, '41a, '41b) have figured prominently in these investigations. Since completing the studies listed above, we have found that histochemically demonstrable phosphatases can react with several phosphorylated substrates other than glycerophosphate (Dempsey and Deane, '46), and that the histological localization characteristic for one substrate may differ from that observed when another phosphate is used. This information has prompted us to re-examine the localization of phosphatases in mammalian placentas, using several substrates rather than the single one previously employed. We have also had occasion to apply