When mercury is added to nitric acid, kept continuously stirred, in the range of concentrations of 4N to 7N, the dissolution of the metal starts after an interval of time which may be designated the period of passivity. This period of passivity is greater in more ditute solutions than in more concentrated ones for a given rate of stirring. The period of passivity in nitric acid solutions of a given concentration depends upon the rate of stirring, being greater in more rapidly stirred solutions. The period of passivity is marked by the accumulation of nitrous acid in solution, until the con­centration of nitrous acid at 22° reaches near about 0.0012 N whereafter dissolution of mercury commences. The period of passivity disappears if the nitric acid contains initially added nitrous acid in sufficient quantities. After the period of passivity, the rate of dissolution is greater in more concentrated solutions than in less concentrated ones at a constant rate of stirring. In nitric acid solutions of the same concentration, the rate of dissolution slows down with increase in the rate of stirring. A mechanism has been suggested for the dissolution of mercury in nitric acid. Nitrous acid is assumed to be the dissolving agent. The initial formation of nitrous acid is assumed to be due to the catalytic decomosition of nitric acid on the surface of mercury, the mercury surface forming an oxygen adsorption complex. The period of passivity has been explained as due to the retarding influence of nitric acid on the dissolving effect of nitrous acid and the period ending with the attainment of a critical concentration of nitrous acid in the reaction mixture.