The effect of Pr and Gd doping on the transport properties of Cu-deficient YBCO superconductors has been studied. Two series of Y₁Ba₂Cu_{3 - x}R_xO_{7 - δ}, where R = Pr or Gd, were prepared by the conventional solid-state reaction technique. Resistance measurements showed a suppression of T_c with increasing of Pr- and Gd-contents in addition to a normal-state metal-to-insulator transition. Moreover, a superconductor-to-insulator transition has been observed at ambient pressure for temperatures less than 50 K for Pr with x = 0.3 and for Gd with x > 0.3. The overall complex behaviours of the resistivity data have been preliminary explained in terms of localization of charge carriers, structural disorders, and magnetic ordering of magnetic moments.