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An earlier proposed endoperoxide intermediate in the biosynthesis of prostaglandins was detected in short-time incubations of arachidonic acid with the microsomal fraction of homogenates of sheep vesicular glands. Conversion of the endoperoxide into prostaglandin E(2) was stimulated by reduced glutathione but suppressed by p-mercuribenzoate and N-ethylmaleimide. The methyl ester of an unknown compound was isolated by solvent extraction and thin-layer chromatography after short-time incubation of arachidonic acid with the microsomal fraction and p-mercuribenzoate. This derivative was identical to the methylester of the endoperoxide, as shown by its conversion into the methyl esters of 11-dehydroprostaglandin F(2alpha) and prostaglandin E(2) by spontaneous rearrangement and its conversion into the methyl ester of prostaglandin F(2alpha) by mild chemical reduction. The smooth muscle-stimulating activity of the endoperoxide ester on the isolated rabbit aortas trip was 4- to 8-times higher than that of the methyl ester of prostaglandin E(2).