We describe a technique for achieving fast volume ray casting on parallel machines, using a load balancing scheme and an efficient pipelined approach to compositing. We propose a new model for measuring the amount of work one needs to perform in order to render a given volume, and use this model to obtain a better load balancing scheme for distributed memory machines. We also discuss in detail the design tradeoffs of our technique. In order to validate our model we have implemented it on the Intel iPSC/860 and the Intel Paragon, and conducted a detailed performance analysis.