The structure of 3',5'-thymidine diphosphate bound in the active site of staphylococcal nuclease (EC 22.214.171.124) was studied by measuring the relaxation rate enhancement of substrate analog nuclei by a paramagnetic metal ion. The lanthanide ion, Gd(III), was substituted for Ca(II) in the formation of the ternary complex of nuclease-Gd(III)-3',5'-thymidine diphosphate. Measurements were made of the transverse relaxation rates of protons and the longitudinal and transverse relaxation rates of the phosphorus nuclei of the bound nucleotide. Internuclear distances between the metal ion and atoms of the 3',5'-thymidine diphosphate nucleotide were determined from these data by the Solomon-Bloembergen equation. In general, these distances corresponded closely to those determined by previous x-ray crystallography of the thymidine diphosphate complex.These internuclear distances were also used with a computer program and graphics display to solve for metal-nucleotide geometries, which were consistent with the experimental data. A geometry similar to the structure of the metal-nucleotide complex bound to nuclease determined by x-ray analysis was one of the solutions to this computer modeling process. For staphylococcal nuclease, the nuclear magnetic resonance and x-ray methods yield compatible high resolution information about the structure of the active site. However, differences of uncertain significance exist between the two structures.