Engineering individual components of a multi-agent system and their interactions is a complex and error-prone task in urgent need of methods and tools. Prototyping is a valuable technique to help software engineers explore the design space while gaining insight and a "feel" for the dynamics of the system; prototyping also allows engineers to learn more about the relationships among design features and the desired computational behaviour. In this paper we describe an approach to building prototypes of large multi-agent systems with which we can experiment and analyse results. We have implemented an environment embodying our approach. This environment is supported by a distributed platform that helps us achieve controlled simulations.