  We study, at the one loop level, the dominant new physics contributions from
extra dimensions to Z -> b,b(bar), as well as B-B(bar) and K-K(bar) mixing. We
use a model with one extra dimension containing fermions which live in four
dimensions, and gauge bosons and one scalar doublet propagating in five
dimensions. We find that the effect of the infinite tower of Kaluza-Klein modes
in Z-> b,b(bar) is finite and gives a negative correction to
R_b=Gamma_b/Gamma_h, which is used to set a lower bound of 1 TeV on the
compactification scale M_c. On the other hand, we show that the box diagrams
contributing to B-B(bar) and K-K(bar) mixing are divergent and, after proper
regularization, we find that they increase the value of the function S(x_t)
which governs this mixing. The obtained value is perfectly compatible with
available data.
