[en] Soon after the discovery of superconductivity at 92K in YBa₂Cu₃O₇, it was shown that Pr suppressed superconductivity in Y_1-xPr_xBa₂Cu₃O₇ when the concentration of Pr exceeded a critical value X_c of 0.55. Further, the value of X_c depended on the ionic radius of the rare earth (at the Y site), X_c being as low as 0.3 for Nd. Several models have been proposed to account for these data but without much success. In the first part we will review the experimental data obtained since 1987. In the second part of this paper, recent results obtained in the last year on the effect of isovalent Sr substitution at the Ba site on x_c will be presented. In particular, it was shown that X_c could be increased from 0.3 to 0.45 when 50% of Ba was substituted by Sr in Nd_1-xPr_xBa₂Cu₃O₇. Further, heating the samples in argon prior to oxygen annealing increased the T_c by 6 to 15K depending on x. In addition, annealing in high pressure oxygen induced superconductivity in certain insulating samples. The normal state properties including Seebeck coefficient will be reported. Neutron diffraction studies indicated a relative slow variation of apical oxygen distance as a function of x(Pr) in Sr substituted systems. In the third part these data will be discussed in the light of certain theoretical models invoking the effect of Prf-CuO₂ hybridisation. Our data show that Sr retards this hybridisation and in addition stress the important role played by oxygen reordering along the chains in the Pr substituted Ln123 compounds. (author)