1. The reaction of pig heart lactate dehydrogenase (EC 184.108.40.206) with NAD + and lactate to form pyruvate and NADH was followed by rapid spectrophotometric methods. The distinct spectrum of enzyme-bound NADH permits the measurement of the rate of dissociation of this compound. 2. The reduction of the first mole equivalent of NAD + per mole of enzyme sites can also be observed, and is much more rapid than the steady-state rate of NADH production. 3. At pH8 the dissociation of the enzyme–NADH complex is rate-determining for the steady-state oxidation of lactate. At lower pH some other step after the interconversion of the ternary complex and before the dissociation of NADH is rate-determining. Other evidence for a compulsory-order mechanism is provided.