1. 1. UDP- N-acetylglucosamine 2-epimerase, the enzyme catalyzing the formation of UDP and N-acetylmannosamine from UDP- N-acetylglucosamine, was purified 100–200-fold from rat liver. The final enzyme preparation was extremely unstable with activity completely disappearing within a few hours. 2. 2. The epimerase reaction had double pH optima of 7.1 and 7.9 and a K m of 0.2 mM for UDP- N-acetylglucosamine was found. 3. 3. The allosteric inhibition of the enzyme by CMP- N-acetylneuraminic acid was investigated in detail. A sigmoidal inhibition curve was obtained suggesting cooperative homotropic effects. A Hill plot yielded an interaction coefficient of n = 4 . 4. 4. The regulatory significance of the epimerase reaction to nucleotide sugar metabolism is discussed.