Considering that the interest rate is the only monetary instrument, we characterize the rules for stabilizing prices and output in a rational expectations model with wages' indexation. The optimal rules depend on the relative information of government and private agents. For instance, the interest rate must be positively related to the price level if the government can identify supply and demand shocks. Indexing the interest rate on the inflation rate or on the rate of growth of the money stock leads to hyperinflation or has destabilizing effects on prices. At least, the knowledge of the money stock, whether it gives an information advantage to the government or not, does not lead to simple monotonic rules. Indeed, the sign of the link between the interest rate and the money stock depends on the parameters of the money demand, on the nature of the shocks, on their stochastic characteristics and on the government final targets, prices or output. This proves the danger of money targetting. The rational expectations hypothesis does not globally alter the results of a standard keynesian model.