The role of the SO(2, 1) symmetry in General Relativity is analyzed. Cosmological solutions of Einstein field equations invariant with respect to a space-like Lie algebra Gr, with 3 ≤ r ≤ 6 and containing so(2, 1) as a subalgebra, are also classified.PACS numbers: 04.20.-q, 02.20.-a, 98.80.-k Gravitational models are usually classified in terms of their group of isometries, namely the number of Killing vectors, the values of the structure constants and the transitivity regions, i.e. the regions on which the isometry group acts transitively. The classification of all non-isomorphic isometry groups is a well known and solved issue in the context of Group Theory; for instance, there exist 9 non-isomorphic 3 -dimensional groups, G 3 , which yield the so-called Bianchi models.A simpler, but important, example is provided by the G 2 groups. In this case the corresponding Lie algebra G 2 is described in terms of two Killing vectors X, Y satisfying the commutation relation [X, Y ] = sY with s = 0, 1, this corresponding respectively to the Abelian and non-Abelian case.Interesting gravitational fields are represented by metrics possessing a higher degree of symmetry (for an exhaustive review on the various classifications of such models available in the literature see for example [11]). Among them particular attention have attracted homogeneous or hypersurface-homogeneous models like the Friedmann-Robertson-Walker metric (FRW) which is G 6 -symmetric, 1