<p> In vivo and in vitro studies on the biosynthesis of vasopressin in the supraoptic nuclei of the dog and guinea pig1 using 35s-cysteine and 3H-tyrosine have suggested that vasopressin could be synthesized by wa;y of a precursor, which is modified to release active hormone. In vivo injection of 3H-tyrosine into the cerebrospinal fluid of rats2 had resulted in incorporation of the label into both oxytocin and vasopressin. In this work, an attempt was made to develop an in vitro system for the biosynthesis of oxytocin. Incubations of either 3H-isoleucine or 14c-leucine with rat hypothalamic neuronal perikaya, ribosomes and cell sap, cell sap, fractions of cell sap and homogenate, and incubations of 3H-isoleucine and l4C-leucine with rat hypothalamic tissue fragments were analyzed for the incorporation of label into purified hormone. Gel filtration, partition chromatography, high voltage electrophoresis, and thin layer chromatography were applied, followed by measurement of radio-activity and biological activity. It is concluded that in no reproducible case was either radioactive isotope incorporated into material with the chromatographic and biological properties of oxytocin. Other radioactive products of incubation were detected in hypothalamic cell sap, ribosomes and cell sap, and homogenate. In hypothalamic homogenate incubations, considerable degradation of both oxytocin and other material absorbing at 280 nm was observed. It is suggested that future investigations should attempt to first develop isolation procedures for the labelled hormone produced in vivo, and then reduce the complexity of the system in small stages, through the cultured hypothalamic-neurohypophyseal system of Sachs3 to simpler in vitro systems. </P> / Thesis / Master of Science (MSc)