Recent changes in the U.S. financial system have made the traditional definition of the adjusted monetary base outdated. Statutory reserve requirements no longer play a significant role in regulating banks' deposit and lending activities. Instead, banks' interbank payments now drive their demand for Federal Reserve Bank deposits. To accurately measure the monetary source base, all Federal Reserve deposits held by domestic banks should be considered, not just those for meeting reserve requirements. Adjustments for changes in reserve requirements need to acknowledge that many banks' behaviors are not influenced by these requirements.