Abstract: In previous paper [ Rodrigues et al (1)], we characterized a space time theory T as the theory of a species of structure in the sense of Bourbaki (2). We succeded in this way in giving precise mathematical and physical meaning to the concepts, of covariance and invariance of T and in introducing the fundamental notion of equivalent reference frames, which are time-like vector fields XE TU, UCM, where M is part of the space-time substructure ST(1,2) for each TE ModT. In particular we showed in the quoted reference that a space-time theory T admits a Principle of Relativity only if GyT (the equivalence group of the class of reference frames of kind X according to T) is different from the identity for some X. Here, after remembering the definition of reference frames appropriated for relativistic space-time [Rodrigues and Faria-Rosa (3) and also (1)], we prove that there are models of General Relativity with a canonically privileged reference frame (cprf). The precise meaning of the cprf is given through Propositions 3 and 4. We show that the cprf can be physically distinguished from any other reference frame with the performance of mechanical experiments (Proposition 4). Although the predicted effects are perhaps very small to be detected within present technology, our results show that no Principle of General Relativity (meaning Physical equivalence of all reference frames) holds for General Relativity. Of particular importance is that even locally inertial reference frames are not equivalent Ver menos