Streaming multimedia applications, such as video on demand or conferencing are increasingly deployed in heterogeneous and mobile environments including Workstations, PDAs, mobile phones, etc. These applications are very resource demanding and in general, they need to be dynamically adapted when executed on low capability terminals. The proxybased content adaptation approach is well suited to transparently adapt in real time multimedia data on intermediate nodes without modifying the application. In this paper, we report on experiments on dynamic configuration of such proxies by using a configuration language called APSL (Adaptation Proxy Specification Language). We developed a configurable proxy allowing adaptation of existing videoconferencing applications, and evaluated the performance benefits of the proxy approach using a DirectShow/COM-based framework.