Experience has shown that prototypingis a valuabletechnique in the validation of designs. However, the prototype(s) can be too far semantically removed from the design. Animation is a technique where a design itself can be `executed' without the need to translate to a high-level language to build a prototype. While animation has been implemented with formal specification languages such as VDM, Z and B and used with some success, we feel that its application to a more graphical specification language/notation would introduce animation to a wider range of software designers. This paper discusses the basis of a technique for the animation of rigorously specified object-oriented models written using the Unified Modelling Language and the Object Constraint Language.