Аннотация:The form factors and widths of the semileptonic decay processes D --> K(K *) l-nu-1, B --> D(D *) l-nu-1, and D(s) --> phi-l-nu(l) are calculated in the quasipotential quark model taking into account relativistic corrections. The calculated form factors of the semileptonic decay D --> K *l-nu(l) agree with experimental data recently obtained by the E691 group at Fermilab. The calculated widths of the decays D --> K(K *) l-nu(l), and B --> D(D *)l-nu(l) are also in good agreement with experiment. The value \V(cb)\ = 0.041 +/- 0.006 is obtained for the Cabibbo-Kobayashi-Maskawa (CKM) matrix element.