An often implemented communication primitive is the remote procedure call (RPC). RPC has the goal of hiding distributed environments from users. To meet the goal, an RPC mechanism must be fast and easy to use. A description is given of an implementation of the RPC primitive in the kernel of the Xinu operating system. The intent of this mechanism is to provide a very efficient and user-transparent interface. The authors present the design decisions, detailed algorithms, and data structures of their implementation. They also discuss the project result and future work.