Two patients ( 1 7/12-year-old and 1 11/12-year-old girls) with chronic hypernatremia were studied. Neuroradiological findings and mildine facial defects showed characteristic features of holoprosencephaly. Water deprivation tests showed clear evidence of antidiuretic hormone (ADH) secretion. The responses to hypertonic saline infusion and acute water loading were abnormal. In one case, the impaired osmotic regulation of ADH secretion was demonstrated by measuring urinary ADH by a radioimmunoassay. In this case, the volume regulation of ADH secretion seemed to be also incomplete since chronic water loading for a period of six days induced water retension.