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1. The preparation of a hormone-binding protein fraction (neurophysin) from porcine neurohypophyses is described.

2. It was shown by gel-filtration that the protein (which sedimented as a single component on analytical ultracentrifugation) forms complexes with oxytocin and lysine vasopressin. The maximum capacity of porcine neurophysin to bind oxytocin and lysine vasopressin was estimated from the results of dialysis experiments as 232 u. oxytocin and 48 u. lysine vasopressin/mg protein.

3. Binding of oxytocin and lysine vasopressin by neurophysin was completely inhibited in the presence of calcium in concentrations greater than 10^-6 M but is unaffected at less than 10^-7 M-Ca. The concentration of calcium required to inhibit binding did not appear to be dependent on hormone or protein concentrations.

4. No evidence was obtained that calcium was bound by neurophysin or by the peptide hormones either alone or in combination.

5. These results are discussed in the light of the hypothesis that release of the hormone from the neurohypophysis is mediated by calcium.