This paper describes the design of a scientific prototyping environment in Smalltalk and discusses implementation strategies to achieve high performance interactive modeling of computationally intensive physical problems. Classes for scientific visualization, including contour plotting and 3D surface representations which incorporate the model-view-controller paradigm are presented. Techniques for inclusion of user primitives written in C to support computationally intense methods are described in detail in their current implementation on advanced Smalltalk workstations (SUN4, Ardent Titan, Tektronix 4317).