We present perturbational ab initio calculations of the leading-order one-electron relativistic contributions to the nuclear magnetic resonance shielding tensor based on the Pauli Hamiltonian. The scalar relativistic and spin-orbit interaction effects, including both relativistic corrections to the wave function ("passive" relativistic effects) and relativistic magnetic perturbation operators ("active" effects), are considered for H2X (X=O, S, Se, Te, Po), HX (X=F, Cl, Br, I, At), and noble gas (Ne, Ar, Kr, Xe, Rn) systems. The perturbational corrections are calculated using linear and quadratic response theory applied to nonrelativistic reference states. We use the uncorrelated self-consistent field as well as correlated, multiconfigurational complete active space self-consistent field models. Results for the H-1 and heavy-atom shielding constants and anisotropies are compared with Dirac-Hartree-Fock and quasirelativistic data. (C) 2003 American Institute of Physics.