The pituitary glands of 11 rats (six females and five males, weighing 140\p=n-\200g.) with hereditary hypothalamic diabetes insipidus (DI) were found to contain oxytocin but no vasopressin.The DI rats could not always be distinguished from rats without diabetes insipidus (non-diabetes insipidus, NDI rats) by measurement of their daily urine volume.Stimulation of the neurohypophysis by fall in arterial pressure (haemorrhage, methacholine) or by nicotine, released vasopressin in NDI rats to produce a sustained antidiuresis, but in the DI rats there was only a transient fall in urine flow which did not outlast the hypotension.The DI rats were about twice as sensitive to vasopressin as NDI rats. As assay preparations they have the advantage that they cannot respond to vasopressin-releasing stimuli.Prolonged administration of Pitressin tannate in oil increased maximum urinary osmolality in DI rats, but failed to increase their sensitivity to intravenously injected vasopressin.