  Decays that are highly suppressed in the standard model are excellent places
to search for effects of new physics. Decays mediated by flavor-changing
neutral currents are forbidden at tree level in the SM, and are often further
suppressed by helicity and the Glashow-Iliopoulos-Maiani mechanism. Exclusive
final states with charged lepton pairs are a particular strength of the CDF
experiment due to the large bottom and charm production cross section and the
ability to efficiently separate signal from background. CDF has searched for
and set the world's best limits on the rare flavor-changing neutral current
decays B^0_(s) rightarrow mu^+mu^-, B^0_(s) rightarrow e^+ e^-, and D^0
rightarrow mu^+mu^-, and the lepton flavor violating decay B^0_(s) rightarrow
e^+/- \mu^-/+.
