We study the process of gauge symmetry breaking in superstring-inspired models with two, one and no extra Z 0' gauge bosons. We present analytic expressions for the solutions of the renormalization group equations for the masses and couplings of an arbitrary extra U(1) model. A specific model with an extra Z 0' (the only one consistent with a heavy v R) is studied numerically and results for the sparticle spectrum, as well as for the constraints amongst parameters, are given. We also consider a minimal “stringy” model with no extra Z 0's. Consistent gauge symmetry breaking in this model requires the t-quark mass m t ≳ 70 GeV, the sneutrino (or a higgsino) being very often the lightest SUSY particle. We argue that the models with an extra Z 0, (although viable), are rather artificial and emphasize the importance of obtaining compactification schemes leading to rank = 4 low-energy groups.