11-Deoxy-[21-3H]corticosterone ([21-3H]DOC) exchanged tritium with water in a reaction mediated by an enzyme preparation obtained from hamster liver. Both (21S)-[21-3H]- and (21R)-[21-3H]DOC participated in the exchange. Incorporation of tritium into water followed a hyperbolic path with time for the 21S epimer and a sigmoidal path for the 21R epimer. Separate enzymes did not appear to be involved. The system was also studied by measuring incorporation of deuterium from deuterated water into the steroid side chain. During incubation, DOC containing 2, 1, and no deuteria were all found. Their proportions changed with incubation time. The deuterium atoms were incorporated into the side chain sequentially and reversibly. It is proposed that (21S)-[21-3H]DOC directly exchanges tritium with water in a reaction catalyzed by an isomerase, and (21R)-[21-3H]DOC loses tritium only after prior enzyme-mediated epimerization to the 21S form. We had previously presented evidence that the isomerase reversibly converted the ketol side chain to the aldol form. Epimerization of the aldol isomer, 20 beta-hydroxy-3-oxopregn-4-en-21-al (isoDOC) by hamster liver enzyme to the 20 alpha-hydroxy isomer was shown directly. A model is presented which accounts for the kinetics of exchange of (21R)- and (21S)-[21-3H]DOC with water, interconversion of DOC and isoDOC, and the epimerization of isoDOC at C-20.