The objective of monetary policy under the inflation target regime is to keep inflation and output at levels that foster a stable economic environment. To reach its objective, the monetary authority uses a policy instrument (such as the overnight interest rate) to achieve the inflation and output targets through the transmission mechanism. However, the transmission mechanisms of monetary policy are not deeply known. In this paper we try to estimate the effects that interest rates fluctuations have on private investment in Brazil. Using firm-level data we use econometrics for panel data and find evidence that monetary contractions have significant negative impacts over the private corporate investment in Brazil. We also tried to estimate how the National Development credit policy interfere in this transmission channel but find little evidence that it does.