Poly(A)+ RNA isolated from post-mortem human hypothalami has been used to characterize the poly-protein precursors to vasopressin and oxytocin. Translation in a cell-free system and subsequent immuno-precipitation with antibodies raised against either vasopressin or neurophysin identified a product of Mr 19000 (prepro-vasopressin). A second less intense product of Mr 16500 was tentatively identified as prepro-oxytocin. A cDNA library derived from the human hypothalamic poly(A)+ RNA was screened for vasopressin and oxytocin-encoding cDNA using heterologous probes; clones encoding the two precursors were identified and found to be organized as their rat and bovine counterparts. Northern blot analysis shows that the mRNAs for the two prepro-hormones consist of approximately 840 (AVP) and approximately 700 (OT) nucleotides.