Since the adoption of the fully-fledged inflation targeting (IT) regime by an important group of central banks, a measure of both the potential output and the natural interest rate have become one of the main concerns of the research agenda. Estimation of the natural interest rate (NIR) is crucial to capture the stance of the monetary policy. In particular, the gap between the instrument rate of the Central Bank and the NIR can be a useful guideline for the position of the monetary policy and can also help to rationalize policy decisions. In this paper we estimate the NIR for the Peruvian Economy. We do so by applying the Kalman Filter to a semi-structural small open economy model with Peruvian data during he sample 1994-2005. Overall, our findings show a persistent reduction on the Peruvian NIR since 1999, which is related to an improvement on the terms of trade and a reduction on the international interest rate. Moreover, the estimated gap shows a loose monetary impulse between 1994 and 1997, tight between 1998 and 2001, and slightly loose between 2002-2005. Finally, the variance decomposition shows that 25 percent of fluctuations in the gap are explained by fluctuations in the NIR. According to this, a time-fixed NIR would give a quite imprecise measure of monetary policy stance.