Usual techniques for automatic test data generation are based on the assumption that a complete oracle will be available during the testing process. However, there are programs for which this assumption is unreasonable. Recently, Chen et al. [3, 4] proposed to overcome this obstacle by using known relations over the input data and their unknown expected outputs to seek a subclass of faults inside the program. In this paper, we introduce an automatic testing framework able to check these so-called metamorphic relations. The framework makes use of Constraint Logic Programming techniques to find test data that violate a given metamorphic-relation. Circumstances where it can also prove that the program satisfies this relation are presented. The first experimental results we got with a prototype tool build on the top of the test data generator INKA, show that this methodogy can be completely automated.