The paper builds a simplified model describing the economy of a currency union with decentralised national fiscal policy, where the main features characterising the policy-making are similar to those in EMU. National governments choose the size of deficit taking into account the two main rules of the Stability and Growth Pact on public finance. Unlike previous literature the asymmetric working of those rules is explicitly modelled in order to identify its impact on the Nash equilibrium of deficits arising from a game of strategic interaction between fiscal authorities in the union.