  The dependence of the pseudoscalar meson masses and decay constants on sea
and valence quark masses is compared to next-to-leading order (NLO) Chiral
Perturbation Theory (ChPT). The numerical simulations with two light dynamical
quark flavors are performed with the Wilson-quark lattice action at gauge
coupling beta=5.1 and hopping parameters kappa=0.176, 0.1765, 0.177 on a 16^4
lattice. O(a) lattice artifacts are taken into account by applying chiral
perturbation theory for the Wilson lattice action. The values of the relevant
combinations of Gasser-Leutwyler constants L_4, L_5, L_6 and L_8 are estimated.
