One of the principle advantages of Asynchronous Transfer Mode (ATM) access is that variable rate streams can be supported efficiently. In order to preserve this efficiency when using ATM on an underlying circuit switched network, such as primary rate ISDN, flexible control of the underlying circuit bandwidth must be provided. This paper discusses techniques used to provide dynamic management of variable rate circuits to support ATM access. Models are presented for some of these control schemes and their performance is studied through a combination of simulation and analysis. Techniques implemented for an experimental network providing ATM access on primary rate ISDN are presented.