In a Model-Driven Development context (MDE), model transformations allow memorizing and reusing design know-how, and thus automate parts of the design and refinement steps of a software development process. A model transformation program is a specific program, in the sense it manipulates models as main parameters. Each model must be an instance of a "metamodel", a metamodel being the specification of a set of models. Programming a model transformation is a difficult and error-prone task, since the manipulated data are clearly complex. In this paper, we focus on generating input test data (called test models) for model transformations. We present an algorithm to automatically build test models from a metamodel.