The one-loop effective potential of the SU(5) model is investigated both in high and low temperature approximation. We find the regions of values of coupling constants and temperatures where the SU(5), the SU(4)×U(1) and the SU(3)×SU(2)×U(1) symmetric states are metastable. A general method of such an investigation is proposed. We observe that the domain structure of the Universe with the simultaneous existence of the gauge symmetries SU(4)×U(1) and SU(3)×SU(2)×U(1) in different domains could take place. © 1982 Springer-Verlag.