Two posterior pituitary hormones oxytocin and arginine-vasopressin control the important activities of water excretion, parturition and lactation. Both these hormones are synthesized as inactive precursors in the hypothalamus along with their carrier proteins neurophysin I and neurophysin II respectively and are activated upon transport to posterior pituitary. Human genes for both oxytocin-neurophysin I (OXT) and arginine-vasopressin-neurophysin II (ARVP) are cloned and found to be linked on chromosome 20 separated by approximately 12 kb of intergenic sequences. Though OXT is not yet associated with any disease, ARVP is linked to the autosomal dominant disease neurohypophyseal diabetes insipidus (AD-NDI). We have mapped regionally the OXT locus to chromosome 20p13 by both radioactive (ISH) and fluorescence in situ hybridization (FISH).