  Constraints from flavour changing neutral currents are discussed in the
context of predictive Grand Unified Supersymmetric Theories. Uncertainties in
the estimation of their amplitudes are minimized by using a successful mass
matrix ansatz which predicts the arbitrary parameters of the Standard Model.
Furthermore, Renormalization Group Equations are used in order to express the
sparticle masses involved, in terms of the gaugino mass $m_{1/2}$, the
universal scalar mass $m_0$ and the trilinear coupling $A$ at the GUT scale.
Modifications in the low energy expressions for the squark masses due to a
large top Yukawa coupling are also considered. The above results, reconciled
with experimental limits are used to put lower bounds on the parameters
$m_{1/2}$, $m_0$ and the low energy scalar masses.
