The difficulties of extending to relativity the results obtained in a preceding paper are set forth; in particular the relativistic behaviour of the time and length minimal constants brings us to a well-known problem. The proposed views attempt to deal with these equations. They give a relativistic generalization of the classical diffusion theory, from which the Klein-Gordon equation is deduced. Consistent results are obtained by a relativistic survey of Dirac's fluid model. Subsequent investigations have been carried out concerning the character of relativity and the limitation of its validity in microphysics.