A circular derivative of bovine pancreatic trypsin inhibitor (c-BPTI) with the C- and N-termini covalently linked with a peptide bond, has been studied in solution by spectroscopic techniques, principally two-dimensional 1H NMR. Near complete sequence specific assignment of the 1H NMR spectrum of c-BPTI has been obtained in a highly efficient manner, using recently developed experimental techniques. The assignments serve as the necessary background for detailed examination of the effects of the chemical modification on global and local features of the protein conformation and on the internal mobility. Analysis of chemical shifts, spin-spin coupling constants, amide proton exchange and proton-proton Overhauser enhancements indicates that perturbations of the native BPTI conformation in c-BPTI involve exclusively residues adjacent to the modification site. The strict localization and small magnitude of the conformation changes relative to native BPTI emphasize the important role of the salt bridge between the chain ends in determining the solution conformation and dynamics in the chain terminal regions of the native protein.