Circular dichroism studies of the bovine neurophysins in the near ultraviolet show a strong negative band at 280 nm and a strong positive band at 248 nm, both of which are attributable almost exclusively to disulfide transitions. The ellipticities per disulfide bond of the unresolved bands in neurophysin-II are -2900 deg cm2/decimole and +2300 deg cm2/decimole at 280 nm and 248 nm, respectively. Binding of oxytocin, vasopressin, or the peptide S-methyl-L-cysteinyl-L-tyrosyl-L-phenylalanine amide lead to large changes in optical activity in the near and far ultraviolet. Of these circular dichroism changes above 290 nm are attributed to changes in the optical activity of neurophysin disulfides, while changes elsewhere are more generally ascribed to changes in either disulfide, tyrosine, or peptide bond transitions. Optical rotatory dispersion studies show that calcium ion, at concentrations of 0.01 M, has only trivial effects on the affinity of bovine neurophysins for oxytocin.