Pre-steady-state studies of the isocitrate dehydrogenase reaction show that the rate constant for the hydride-transfer step is above 900s-1, and that both subunits ofthe enzyme are simultaneously active. After the fast formation of NADPH in amounts equivalent to the enzyme subunit concentration, the rate of NADPH formation is equal to the steadystate rate if the enzyme has been preincubated with isocitrate and Mg2+. If the enzyme has been preincubated with NADP+ and Mg2+, in 0.05M-triethanolamine chloride buffer, pH 7.0, with the addition of0.1 M-NaCl, the amount of NADPH formed in the fast phase is only 60 % of the enzyme subunit concentration, and the turnover rate is at first lower than the steady-state rate. In 0.05M-triethanolamine chloride buffer, pH 7.0, if the enzyme is preincubated with NADP+ or NADPH, the turnover rate increases 3-fold to reach the steady-state rate after about 5s. Preincubation of the enzyme with isocitrate and Mg2+ abolishes this lag phase, the steady-state rate being reached at once. It is suggested that the enzyme exists in at least two conformational forms with different activities, and that the lag phase represents the transition (k = 0.4s-1) from a form with low activity to the fully active enzyme, induced by the binding of isocitrate and Mg2+.Isocitrate dehydrogenase [threo-D%-isocitrate-