Proton magnetic resonance spectroscopy was used to monitor individual amino acid residues in bovine neurophysin, in the nonapeptide hormone oxytocin, and in the complex formed between them. For neurophysin I alone, a normal titration curve for the C-2 proton resonance of the lone histidine residue was obtained with an apparent ionization constant of 6.9 addition of oxytocin to a solution of neurophysin I at pH 6.5 resulted in several changes in the spectrum. The effect on the histidine C-2 proton resonance signal indicated a slow exchange process between two states, probably representing a conformational change in the protein. The apparent pK of the histidine residue in the hormonal complex was shifted to 6.7, indicating a slightly more positive (less electron dense) environment for the histidine residue. Resonances of the single tyrosine residue of oxytocin were observed to broaden significantly, but not to shift appreciably, on the addition of neurophysin II. These observations may indicate involvement of the tyrosyl residue of oxytocin in the hormone-"carrier protein" interaction.