The rate for the final state radiation of hadrons in leptonic
$Z$ decays is evaluated, using as input experimental data for
$\sigma (e^+e^-\to hadrons)$ in the low energy region. Configurations
with a lepton pair of large and a hadronic state of low invariant
mass are dominant. A relative rate
$\Gamma_{l\bar l had}/\Gamma_{l\bar l}=6.3\times 10^{-4}$ is calculated.
This result is about twice the prediction based on a parton model
calculation with a quark mass of $300$ MeV.
The rate for secondary production of heavy quarks is calculated in the
same formalism.
