The magnetization and magnetoresistance of Ln_1-xBa_xMnO₃ have been studied for a single crystal (Ln = Nd, x = 0.23) and for ceramics (Ln = Nd, Sm; 0 x0.44). It was shown that Nd_1-xBa_xMnO₃ compositions in the range 0 x0.05 are inhomogeneous antiferromagnets; a mixed antiferro-ferromagnet state appears in the range 0.05<x<0.10; a pure ferromagnetic state develops in the range 0.2 x<0.4 and in the range x0.4 an inhomogeneous ferromagnetic state is realized. Metal-like behaviour below T_C is found in the concentration 0.3<x0.44. The concentrational transition from inhomogeneous antiferromagnetic state into spin-glass state in Sm_1-xBa_xMnO₃ was found at x>0.12. Both Nd and Sm sublattices are coupled antiferromagnetically with manganese ions. All the ferromagnetic Nd_1-xBa_xMnO₃ samples show similar magnetotransport properties despite of a strong difference in the resistivity behaviour. Our results could be understood in terms of Mn-O-Mn superexchange and 3d orbital ordering as well as variation of band structure due to the ionic size effect, Mn⁴⁺ ion appearance and magnetic ordering. It is supposed that the gap between the narrow 3d manganese band and wide 2p oxygen band decreases strongly below T_C.