A chromophorics and fluorescent analog of uridine 5'-monophosphate (UMP), a known competitive inhibitor of UDPglucose 4-epimerase was synthesised. This analog, namely 2',3'-O-(2,4,6-trinitrocyclohexadienylidene) uridine 5'-monophosphate, was found to be a powerful reversible inhibitor of UDPglucose 4-epimerase indicating its interaction with the substrate binding site of the enzyme. The extreme sensitivity of the fluorescence emission spectrum of this analog to solvent polarity makes it an excellent probe for the study of the environment at the active site of the enzyme. We report here the effective use of this UMP analog to demonstrate that the hydroxyl groups of the ribose moiety of UMP and presumably the substrates (UDPgalactose and UDPglucose) do not reside in a hydrophobic milieu.