Different monetary aggregates covary very differently with short term nominal interest rates. Broad monetary aggregates like Ml and the monetary base covary positively with current and future values of short term interest rates. In contrast, the nonborrowed reserves of banks covary negatively with current and future interest rates. Observations like this 'sign switch' lie at the core of recent debates about the effects of monetary policy actions on short term interest rates. This paper develops a general equilibrium monetary business cycle model which is consistent with these facts. Our basic explanation of the 'sign switch' is that movements in nonborrowed reserves are dominated by exogenous shocks to monetary policy, while movements in the base and Ml are dominated by endogenous responses to non-policy shocks.