  Semileptonic decays of heavy baryons consisting of one heavy (Q=b,c) and two
light (q=u,d,s) quarks are considered in the heavy-quark--light-diquark
approximation. The relativistic quasipotential equation is used for obtaining
masses and wave functions of both diquarks and baryons within the constituent
quark model. The weak transition matrix elements are expressed through the
overlap integrals of the baryon wave functions. The Isgur-Wise functions are
determined in the whole accessible kinematic range. The exclusive semileptonic
decay rates are calculated with applying the heavy quark 1/m_Q expansion. The
evaluated Lambda_b -> Lambda_c l nu decay rate agrees with its experimental
value.
