We have compared transmembrane potentials (Em) of maternal liver with Em of fetal liver, and as an initial step to account for differences in Em, we have measured intracellular potassium ion activities (ak ) in both tissues. Paired segments of maternal and fetal (day 17) mouse liver were suffused (15 ml/min) with Krebs' physiologic salt solution equilibrated with 95% 0 2 -5 % C 0 2 (pH 7.3-7.4) at 37OC. To measure Em, cells were impaled with open-tip rnicroelectrodes filled with 0.5 M KCI. lntracellular voltage recordings that were stable k 2 mV for at least I 0 s were considered valid impalements. Maternal liver mean Em = -41 1 (SEM) mV, n = 70 animals. In contrast, fetal liver mean Em = -23 f 1 (SEM) mV, n = 10 animals. In the same segments we measured a;(with potassium-selective liquid ion-exchanger microelectrodes. Maternal liver mean a;(= 95 f 7 (SEM) m M and fetal liver mean a;(= 62 f 4 (SEM) mM. in addition, Em and akof fetal liver increased to values comparable to those of maternal liver during the first 8 days of neonatal life. The differences of Em and ak between fetal and maternal liver, and the changes in these values that occur in the neonate, may result from activity of a membrane Na-K exchange pump that increases with tissue development.