The need to reduce development time and risk for large, complex application programs has driven the concept of rapid prototype programming. This paper describes the results of rapid prototyping a direct numerical control algorithm. Approximately 2800 lines of finished ZetaLISP code (including comments) were generated and tested on a Symbolics 3640 during a three week period. The code includes a primitive geometry sketch pad, the DNC control algorithm, and an output simulation module. It took four days to build a primitive operational system, which suggested how best to proceed with the complete design and implementation. The entire prototyping process seemed very natural and intuitive. Including all phases of design and implementation, sustained rates of over 220 lines of code per day per person can be achieved. This confirms the idea that the Symbolics 3640 environment can improve significantly the productivity of individual programmers. This paper also suggests ways to extend existing code and better ways to develop new code.