Assays of UDP-glucose dehydrogenase at pH 6.0 show long (10-15 min) lag periods before the steady-state rate is established, but at pH 9.0 no lag is observed. At intermediate pH values the lag is progressively shorter as the pH becomes more alkaline. The behaviour of the enzyme in assays at neutral and acid pH depends on the pH and concentration of the enzyme used to initiate the assay. The steady-state rate at pH 6.0 is strongly concentration-dependent. It is suggested that these phenomena arise because of the slow dissociation of an inactive enzyme species to an active one. Purified preparations of the enzyme release approx. 1 mol of a UDP-sugar/mol of enzyme subunit on denaturation. The identity of the UDP-sugar is unknown.