  We evaluate the form factors governing the exclusive decays B->rho l nu,
B->a_1 l nu, by using an effective quark-meson lagrangian. The model is based
on meson-quark interactions, and the computation of the mesonic transition
amplitudes is performed by considering diagrams with heavy mesons attached to
loops containing heavy and light constituent quarks. This approach was
successfully employed to compute the Isgur-Wise form factors and other hadronic
observables for negative and positive parity heavy mesons and is presently used
for exclusive heavy-to-light weak transitions. We also evaluate a few strong
coupling constants appearing in chiral effective lagrangians for heavy and
light mesons.
