There is little information on the placental lipid content of mammals during periods of high energy demands in the course of gestational development. It was therefore considered of interest to study the lipid (mainly phospholipid) content of the developing human placenta. Placental tissues (between 6 and 24 wk of gestation) were collected via routine hysterotomy and term placental tissues were collected at the time of parturation or via Caesarian section. Gestational age was calculated from the period of amenorrhea and crown-rump length of fetus. In the developing human placenta both the total lipid and total phospholipid content are present at highest concentrations at 10-14 wk of gestation and then decrease gradually as the gestation proceeds, which may suggest that lipid may be serving as one of the energy sources. This increase in phospholipid content in early development may also meet the requirements for the formation of cellular and subcellular structures. Of the individual phospholipids, phosphatidyl ethanolamine and phosphatidyl choline (together with phosphatidyl serine, which could not be separated in one-dimensional TLC but was identified by two-dimensional TLC; however, PS, Pl and PC could be separated in another solvent system, chloroform: methanol: acetic acid: water, 60:36:6:3) vary in more or less the same way during the development of the placenta. The levels decrease from early to mid-gestation, increasing slightly again at term. However, sphingomyelin content increases rapidly from early to mid weeks declining at term.