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An evaluation of interactions between macroeconomic activity and fiscal policy measured by a balance variable proves disappointing if carried out using a linear reduced-form VAR model. This study adopts an alternative representation, based on Hamilton’s work, which allows a model’s coefficients to vary according to the phase of the cycle and the fiscal system. The effect of fiscal policy on activity is thus reappraised for four OECD countries, first as a function of the cyclical phase and then of mixed systems in relation to cyclical conditions and fiscal policy trends. In several cases, non-neutral, classical or Keynesian effects on the budget were found to be specific to certain systems, although they did not emerge in a linear model. The American situation seemed to be closest to traditional a priori macroeconomics, with significant Keynesian effects in recession.