The sustainability of the public finances is indispensable to get the macroeconomic stability. The fiscal sustainability is reached when the public debt as percentage of the PBI stays constant in a level considered appropriate, or it decreases gradually of a level considered inadequate. In this paper establish a theoretical model about on the sustainability of the public debt in a small and open economy, in a context of free mobility of capitals. To the basic framework theoretical, was incorporated, two interest rates, one for the internal public debt and another for the external public debt and, it supposed, also, the compliance of the parity not covered of interest rates. This way, the model captures two market risks, the associated to the variations of the international rates interest, of the exchange rate or of the expectations of devaluation. The model shows that, in an institutional mark where the authorities have difficulties to reduce the expenses or to elevate the taxes, that is to say, for a given level of primary balance, a reduction of the rate of growth of the product and an increment of the rate of devaluation of the exchange rate or of the external interest rate, increase the coefficient of public indebtedness. On the other hand, in a scenario where is possible to reduce the expenses or to elevate the tributes, before the presence of these events, when the authorities decide to maintain constant the coefficient of public indebtedness, the necessary primary balance for this objective has to rise.
