By trading off temporal and spatial quality with available bandwidth, or manipulating the playout time of continuous media in response to variation in delay, audio and video flows can be made to adapt to fluctuating network conditions with minimal perceptual distortion. In this article the authors describe the implementation of an adaptive transport system that incorporates a QoS-oriented API and a range of QoS mechanisms that best assist multimedia applications in adapting to fluctuations in the delivered network QoS. The system, which is an instantiation of the transport and network layers of a QoS architecture, is implemented in a multi-ATM switch network environment with Linux-based PC end systems and continuous media file servers. A performance evaluation of the system configured to support a video-on-demand application scenario is presented and discussed. A novel aspect of the system is the implementation of a ''QoS adaptation'' algorithm which allows applications to delegate to the transport system responsibility for augmenting or reducing the perceptual quality of video and audio flows when network resource availability increases or decreases, respectively.