The prostaglandin-E, 9-reductase (PGE, 9-reductase) activity in the corpus luteum of rabbits corresponds to a cytosolic, NADPH-dependent enzyme with a molecular mass of 36 kDa. This enzyme was purified from corpora lutea on day 12 of pseudopregnancy with a 266-fold enrichment. The main purification step was affinity chromatography using Red Sepharose CL-6B. The efficiency of this column was improved by elution with 1 mM NADH prior to elution of the active fractions with 1 mM NADPH.Amino acid sequence data demonstrate that the rabbit luteal PGE, 9-reductase has to be classified as a member of the aldo-keto reductase superfamily. The enzyme revealed a wide substrate specificity comprising the reduction of aldehydes, ketones, and quinones. Apparent kinetic constants were determined using methylglyoxal, DL-glyceraldehyde, and 9,lO-phenanthrenquinone as substrates. The fully purified enzyme showed two catalytic activities of particular interest: PGE, 9-reductase and 20a-hydroxysteroid dehydrogenase (20a-HSD) activities. The competitive inhibition of 20a-HSD activity by PGE, indicates that progesterone and PGE, are substrates for the same enzyme. From these results, we conclude that prostaglandin and steroid metabolism are tightly linked to each other. For this reason the aldo-keto reductase could be a key enzyme in the cascade of events leading to the regression of the corpus luteum in the rabbit.