We estimate a DSGE model of the brazilian economy taking into account the transition from the exchange rate band system to inflation targeting occurred in 1999. We use a new Keynesian small open economy model where the Central Bank follows structurally different monetary policy rules in each regime. By comparing the transmission channels of exogenous shocks we find significant differences across the regimes, specially on those shocks related to the foreign sector of the economy.We then perform counterfactual experiments where we simulate the response of key macro variables under alternative scenarios for the regime transition. Among other results our simulations suggest that the continuation of the exchange rate band system could have been unsustainable while anticipating the transition to inflation targeting before the Russian crisis of 1998 could have benefited the economy. Additionally, we show that a structural break test applied to the whole data sample correctly identifies the regime change in 1999.