Аннотация:In the framework of the quasipotential quark model the pseudoscalar decay constants of B and D mesons are calculated with inclusion of relativistic corrections of order upsilon-2/c2. The resulting constants are used to analyze weak leptonic decays of the mesons and B(q)0-B(q)0BAR mixing. The extraction of elements of the Kobayashi-Maskawa matrix from the experimental data is discussed.