Complete cDNA sequences for the vasopressin and oxytocin precursor polyproteins have been determined for the rat, calf, human and pig (vasopressin only), indicating the essential conservation of the precursor structures throughout mammals. DNA probes specific for vasopressin or oxytocin mRNAs have been used to identify both classic (hypothalamic) and novel (thymus, corpus luteum, phaeochromocytoma) sites of hormone expression. Semiquantitative DNA/RNA hybridization suggests that in rats expression of the vasopressin and oxytocin genes is positively effected by osmotic stress, negatively by a systemically applied excess of vasopressin; in the latter experiment a reduction in the hypothalamic levels of vasopressin and oxytocin mRNAs in normal and Brattleboro rats have been observed. This suggests a feedback regulation by the hormone as a possible element in controlling the transcription of the vasopressin gene.