This paper develops a positive model of reserve requirements and interest on reserves, based on the observation that Congress exempted the Fed from a legal restriction that had prevented private clearing houses from issuing their own currency. Eliminating the restriction provided the Fed with a source of revenue that could be used to finance general government outlays and to pay implicit interest on reserves. The model implies that the government's financing requirements help explain reserve requirement movements and that interest rates on reserves vary with market loan rates. Cointegration, error-correction, and Granger-causality tests provide supporting evidence.