mRNA from membrane-bound polysomes of bovine hypothalamus was translated in an mRNA-dependent cell-free system from reticulocyte lysate or wheat germ. The translation products were identified by immunoprecipitation with antibodies to either neurophysin II or arginine vasopressin followed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. An immunoreactive polypeptide was obtained with an apparent Mr of 21,000. Sequential immunoprecipitation studies indicated that the Mr 21,000 product is a common precursor to neurophysin II and arginine vasopressin. The specificity of the immunoprecipitation was demonstrated by competition with excess amounts of unlabeled neurophysin II or arginine vasopressin; little or no competition was observed with unlabeled neurophysin II or arginine vasopressin; little or no competition was observed with unlabeled neurophysin I or oxytocin. Processing experiments with microsomal membranes from dog pancreas or tunicamycin-treated ascites tumor cells showed that the Mr 21,000 polypeptide is the prepro form. It was converted to a pro form with Mr 19,000 suggesting a pre sequence of approximately 15 amino acids. The Mr 19,000 polypeptide was coreglycosylated to an apparent Mr of 23,000, indicating that the neurophysin II-arginine vasopressin precursor is a glycopolypeptide.