The present study provides the first evidence forδ13-reduction of 15-ketoprostaglandins (15-keto-PGs) by bovine ocular tissues. The 9,000xg supernatants of cornea, iris, ciliary body, retina, and RPE-choroid except lens exhibitedδ13-reductase activity toward 15-keto-PG E2and F2αin the presence of NADPH or NADH as an electron donor. Among the tissues tested, the highest activity was observed in ciliary body and iris, followed by RPE-choroid, retina, and cornea. The NADPH- and NADH-linked double bond reductase activities were inhibited by dicumarol, quercitrin, indomethacin and disulfiram, but not by potassium cyanide.NADPH-linked 15-keto-PG F2αδ13-reductase was purified from bovine iris-ciliary body cytosol by fractionation with ammonium sulfate and high performance liquid chromatography (HPLC) with TSK gel DEAE-5PW, and TSK gel Blue-5PW. The purified enzyme was homogenous by the criterion of sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Its molecular weight was estimated to be about 57,000 by electrophoresis, and about 55,000 by gel filtration HPLC with Superose 12.