  A detailed evaluation is presented of production of heavy fermions
(particularly ttbar quarks) by a vector current, in the region around
threshold. This includes bound states as well as the region above threshold, to
the same degree of accuracy; in both cases radiative and nonperturbative
corrections are included. The contribution of this region to the vacuum
polarization (``threshold effects'') is calculated and compared with a
perturbative calculation; the importance of the threshold effects is found
substantially smaller than claimed by some authors. Open fermion-antifermion
production is also discussed.
