An important task in the conceptual modelling process of information systems is the validation of the model. The validation task has the objective of checking whether the model correctly and adequately expresses the requirements informally stated by the users. Different techniques and tools have been developed to support this task. We propose an animation tool for an object-oriented specification language. This tool generates executable specifications from conceptual model specifications on the same level of abstraction. In this way, the model behaviour can be observed and checked against the intended user requirements.