In order to study production of prostaglandins (PGs) by human pre-ovulatory ovarian tissue, bioconversion of 14C-arachidonic acid to PGs was examined using homogenates of human Graafian follicles and stromal tissues fully stimulated by endogenous follicle-stimulating hormone in the regular menstrual cycle. Among the radioactive metabolites extracted, PGE2 and 6-keto PGF1 alpha were purified and identified by silicic acid column-, thin layer-, reversed phase partition chromatographies and radio-gas chromatography. The follicular tissue was shown to convert 14C-arachidonic acid to PGE2 at a conversion rate of 0.34% but not to 6-keto PGF1 alpha. On the other hand, the stromal tissue was demonstrated to produce not only 14C-labeled PGE2 at a conversion of 0.09% but mainly 14C-labelled 6-keto PGF1 alpha at 0.26%.