Mitochondrial lipids and ATPasc activity have been studied in developing human placenta. The enzyme activity in the placenta increases steadily with gestational age until the 18th to 21st week, and decreases in the second half of pregnancy. Both Mg2+ and Ca2+ ions have been found to activate the enzyme system. The solubilization of membrane lipids by deoxycholate inhibits the enzyme activity. Total lipid and total phospholipid contents of the mitochondrial membranes also increase during the early development of the placenta with peaks in the 14th to 17th week, after which the levels decrease. Howevcr, the percentage composition of individual phospholipids remains almost the same throughout pregnancy. The profile of the ATPase activity exhibits a correlation with that of membrane lipids in human placenta during intrauterine development of the tissue. © 1980 S. Karger AG, Basel.