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Changes taking place in metal surfaces during running-in tests are reported. The surfaces were lubricated during the tests which simulated conditions encountered in industrial practice. In all tests the geometry of the surfaces was changed. Furthermore, under certain conditions, metallurgical changes could be observed. The material of the transformation was considerably harder than the martensitic steel of the test samples, and the etching characteristics of the transformed material were markedly different from those of the underlying steel. For initially smooth surfaces, the metallurgical transformations had the form of ridgelike eruptions protruding over the surface of the test specimen by several microns. Different tests for the identification of the nature of the transformation are discussed. Heat-conductivity calculations made it possible to estimate the temperature reached during running-in. A tentative explanation of the origin of the transformations is presented.