The lipid composition of the guinea pig placenta was found to vary with the duration of pregnancy. Between the 20th and the 40th days there occurred an increase in phospholipid and free cholesterol, both of which remained elevated from then on to term. There was no significant change at any time in the amount of cholesterol esters, but that of neutral fat increased steadily sixfold and more during pregnancy. These changes were interpreted as signifying a gradual change in placental lipid metabolism during pregnancy. The relation of this change to the transfer of lipids from mother to fetus, and its relation to the etiology of the lipemia of pregnancy in guinea pigs, are discussed.