  We calculate production by vector and axial currents of heavy quark pairs
($c\bar{c}$, $b\bar{b}$, $t\bar{t}$) close to threshold. We take into account
strong interaction contributions (including radiative corrections and leading
nonperturbative effects) by using the Fermi-Watson final state interaction
theorem. We use the results obtained to compare with experiment for open
production of $c\bar{c}$, $b\bar{b}$ near threshold, and to give a reliable
estimate of the so-called ``threshold effects'' contribution to vector and
axial correlators, for $t \bar{t}$, $i.e.$, the contribution of regions close
to $4 m_t^2 $ to $\Pi(t)$, for small values of $t$ ($ 0 < t
\lower2pt\hbox{$\lesssim$} M_Z^2 $).
