We analyze the sustainability of the government's intertemporal budget constraint and the corresponding fiscal reaction function within a nonlinear error-correction framework. Our empirical analysis, based on Italy, provides some evidence that the Italian government is meeting its intertemporal budget constraint. Nevertheless, we show that the burden of correcting budgetary disequilibria is entirely carried out by changes in the average tax rate, with a weakly exogenous government spending, possibly determined by the political process. We also document some rigidities of the tax instrument, in terms of downward inflexibility of the average tax rate with respect to its long-run level. Finally, we provide some evidence in favor of a nonlinear adjustment toward a sustainable long-run equilibrium, as the average tax rate adjusts faster the further away it gets from the equilibrium. By considering the behavior of taxes across the economic cycle, we also provide some evidence of inflexibility of the tax instrument during bad times. (JEL C32, C51, C52, H20, H50)