THE equality of the gravitational mass and the inertial mass for all kinds of matter and energy—the equivalence principle—was assumed to be a universally valid principle by Einstein in building his relativistic theory of gravitation. This equality has been experimentally verified with great accuracy for single neutrons1, for samples of various materials2 and for the planets3. Because Einstein's theory predicts the correct advance of the perihelion of Mercury, it is usually believed that the equivalence principle is therefore also confirmed for large bodies like the Sun, or the stars, for which no other experimental verifications are at present known. In fact, as will be shown, the astronomical observations cannot exclude a relatively large failure of the principle for the Sun. The hypothesis that such a failure may eventually explain the small non-Newtonian effects observed in the solar system will also be examined.