  We study the deep inelastic structure functions of mesons within the Nambu
and Jona-Lasinio model. We calculate the valence quark distributions in $\pi $,
$K $, and $\rho $ mesons at the low energy model scale, which are evoluted to
the experimental momentum scale in terms of the Altarelli-Parisi equation. The
resulting distribution functions show reasonable agreements with experiment. We
also discuss the semi-inclusive lepton nucleon scattering process with a slow
nucleon in coincidence in the final state, which reveals the off-shell
structure of the pion.
