Uptake and metabolism of tritium labelled arachidonic acid have been studied under in vivo circumstances in placenta, uterus and fetal membranes of pregnant guinea pigs on days 40 and 60 of pregnancy. Distribution of radioactivity within the lipid fractions of the selected tissues showed a characteristic pattern depending on the gestational ages. Composition of labelled lipids was determined by radio thin layer chromatography and quantitated with liquid scintillation measurements. The main site of arachidonic acid incorporation was the 2-position of phosphatidylcholine. Near term, a considerable PGF2 alpha-synthesis from exogenously administered 3-H-arachidonic acid could be demonstrated for the first time "in vivo".