High-resolution 'H nuclear magnetic resonance (NMR) spectroscopy at 300 MHz has been used to study the behavior of human gastrin in aqueous solution. A large number of resonances have been assigned by analysis of one- and two-dimensional NMR spectra and the effects of pH and by comparison with the spectrum of des-<Glu1-gastrin. In gastrin, the ratio of cis to trans conformations around the Gly-2 to Pro-3 peptide bond is 3:7. This is reflected in splitting of the resonances of several neighboring residues and of a residue distant in the sequence, Tyr-12. The pKa of Tyr-12 is 10.7. Sulfation of this residue perturbs the resonances of Tyr-12 and Gly-13 but has very little effect on the rest of the spectrum. A study of the temperature dependence shows that several perturbed resonances move toward their expected positions as the temperature is raised but with a linear dependence on temperature, consistent with a redistribution of populations among accessible local conformations rather than a cooperative conformational change. Addition of Na+ or Ca2+ causes only minor changes in the spectrum. The paramagnetic metal ion Co2+ produces a number of spectral changes, reflecting strong binding to at least one site involving the Glu residues and weaker binding to Asp-16.