We have developed a system for interactive manipulation of shading parameters for three dimensional rendering. The system takes as input user-defined shaders, written in a subset of C, which are then specialized for interactive use. Since users typically experiment with different values of a single shader parameter while leaving the others constant, we can benefit by automatically generating a specialized shader that performs only those computations depending on the parameter being varied; all other values needed by the shader can be precomputed and cached. The specialized shaders are as much as 95 times faster than the original user defined shader. This dramatic improvement in speed makes it possible to interactively view parameter changes for relatively complex shading models, such as procedural solid texturing.