A study has been made of the activity of adenosine deaminase in the placenta and juxtaplacental endometrium (maternal Component) of the cow, cat, guinea pig, and human at intervals throughout gestation. The maternal components of the bovine, guinea pig, and feline placentas were separated from the fetal components and the major source of adenosine deaminase activity was found to be in the maternal components in each species. The enzyme activity in the maternal components of the bovine and guinea pig placentas and in the detached feline placenta increased early in gestation (to peak values of 15, 10, and 58 units/mg protein, respectively) and then gradually decreased until term. The human placenta contained much less enzyme activity and no significant difference in this activity between term and first trimester placentas was detected.