We describe the PVR (Parallel Volume Rendering) system, which consists of a mix of modular C/Tcl/Tk code. With PVR it is possible to balance rendering efficiency, throughput, and latency over a large number of parallel machines configurations. It supports different rendering schemes and it can be easily augmented with others. PVR has unique capabilities: it can be used to build large and complex distributed visualization environments, and it lets the user control the rendering performance. We also discuss interactivity issues for parallel volume rendering environments. In particular, we give detailed performance numbers we achieved using PVR on an Intel Paragon.