Rabbit kidney prostaglandin 9-ketoreductase was found to metabolize the glutathione conjugate of prostaglandin A (GSH-prostaglandin A). Apparent K (GSH-prostaglandin A) 13 µM and apparent K (prostaglandin E) 200 µM. The cytosolic preparation was subjected to gelfiltration and isoelectric focusing, which revealed that metabolism of prostaglandin E and GSH-prostaglandin A occurs by means of the same fractions. Furthermore, prostaglandin E and GSH-prostaglandin A are competitive inhibitors of the enzyme, when GSH-prostaglandin A and prostaglandin E are tested as substrates, respectively. It is concluded, that GSH-prostaglandin A is a much better substrate for prostaglandin 9-ketoreductase from rabbit kidney than is prostaglandin E.