The idea of an exogenous money supply—controlled entirely through central bank interventions—was a fundamental tenet of monetarism and New Classical economics. Post Keynesians have developed an extensive literature arguing that the money supply is in fact endogenous—that market forces combine with central banks in establishing the money supply. But Post Keynesians disagree on a related question: to what extent are interest rates set exogenously by central banks? To address this issue, this paper presents evidence regarding the movement of market interest rates in U.S. financial markets relative to the Federal Reservecontrolled Federal Funds rate. Concluding that market interest rates are primarily set through market forces—i.e. are largely endogenous—the paper then discusses the primary source of interest rate endogeneity. This is the instability of deregulated financial markets, which leads market participants to make wide swings in their risk assessments over time. It follows that effective regulatory policies to stabilize markets and control interest rates directly will increase the degree of interest rate exogeneity. The paper concludes with proposals for establishing greater control over market interest rates.