Bovine neurophysin-II is shown to be very susceptible to partial reduction in the absence of urea. Reduction of an average of one disulfide leads to major changes in conformation and disulfide optical activity, manifest in part by pronounced far-uv ellipticity changes, complete loss of the 248-nm ellipticity band, and a shift of the 278-nm ellipticity band to shorter wavelengths with loss of half its intensity; the reduction process generates a mixture of products and appears to be accompanied by disulfide interchange. The circular dichroism data indicate that the disulfide(s) most susceptible to reduction or interchange are either the principal contributors to the 248- and 278-nm ellipticity bands or that the optical activity of other disulfides is dependent on their integrity. Peptides that bind to the hormone-binding site of neurophysin-II protect against reduction. On reoxidation of partially reduced neurophysin-II there is only a partial return of the native circular dichroism spectrum and electrophoretic behavior. The percentage of native protein in samples reoxidized following different degrees of reduction was estimated by comparison of the circular dichroism spectra of these samples with those of the fractionated native and denatured components of monoreduced-reoxidized neurophysin. Under our reoxidation conditions, less than 50% native protein was found in monoreduced-reoxidized neurophysin and less than 10% native protein was found in completely reduced-reoxidized neurophysin. The results are interpreted with qualified reference to a model in which one or more disulfides are "strained" in the native state and in which the native protein is unstable relative to species in which the disulfides are differently paired.