The interaction between isoniazid and reducing sugars is acid-catalyzed and reversible. Kinetic studies of hydrazone formation from isoniazid and glucose, lactose, maltose, and galactose have been carried out in simulated gastric juice at 37 degrees C. The forward reaction was found to follow second-order kinetics, while the reverse reaction, the hydrolysis of the sugar isonicotinoyl hydrazone, is pseudo-first-order. The effects of the concentration of reactants, pH, and temperature on the rate have been studied, and the rate constants and the energy of activation were determined.