The biosynthesis of prostaglandins by isolated epithelial glandular and stromal cells was studied after collagenase digestion of endometrium collected from women at various stages of the menstrual cycle. Homogenates of the separate cell types were incubated for 60 minutes with 2.08 micrograms 1(14)C arachidonic acid and the products separated by thin layer chromatography. Both glandular and stromal homogenates synthesised PGF2 alpha. More PGF2 alpha was synthesised by glandular epithelium separated from both proliferative and secretory endometrium than by stroma. The ratio of PGF2 alpha/PGE2 was greater in glands and stroma isolated from secretory than proliferative endometrium. Small but significant amounts of 6-keto-PGF1 alpha were produced by all cell types. These results suggest that the increased synthesis of PGF2 alpha from secretory endometrium is due, at least in part, to increased activity of cyclo-oxygenase enzyme in the glandular epithelium.