Two families with hereditary central diabetes insipidus (CDI) are reported. The pedigree in both families shows an autosomal dominant trait. The plasma arginine-8-vasopressin (AVP) determined by radioimmunoassay was markedly lower in these CDI patients than in healthy controls; the difference being even more pronounced after a hyperosmotic challenge. Since in the present study histamine also failed to increase the plasma AVP concentration, the authors consider it unlikely that an osmoreceptor failure would be implicated in the pathogenesis of CDI in these cases. The AVP concentration of the lumbar cerebrospinal fluid was also measured in two members of one of the families: the level found at the lower normal range indicates that some AVP secretion has been maintained in the extrahypothalamic vasopressinergic system of these patients.