  A rainbow-ladder truncation of QCD's Dyson-Schwinger equations, constrained
by existing applications to hadron physics, is employed to compute the
valence-quark parton distribution functions of the pion and kaon. Comparison is
made to pi-N Drell-Yan data for the pion's u-quark distribution and to
Drell-Yan data for the ratio u_K(x)/u_\pi(x): the environmental influence of
this quantity is a parameter-free prediction, which agrees well with existing
data. Our analysis unifies the computation of distribution functions with that
of numerous other properties of pseudoscalar mesons.
