SUMMARY Unit activity was recorded in the supraoptic (SO) and paraventricular (PV) nuclei in Brattleboro (DI) rats both homozygous and heterozygous for the factor causing diabetes insipidus, and from normal Long-Evans rats from which the Brattleboro strain is derived. In these nuclei the distance between cells which could be antidromically excited by stimulation of the neural lobe did not differ between the three groups of animals. The electrical properties of the antidromically excited neurones, as judged by the latency of the antidromic spike, the maintenance of the amplitude of the spikes after short trains of stimuli, the existence of phasically firing units and antidromic inhibition, did not seem impaired in DI rats. The spontaneous discharge rates of antidromically identified neurones in the SO nuclei of both homozygous and heterozygous DI rats, but not of other hypothalamic units, were significantly faster than those in normal animals. Plasma sodium concentration and osmolality are known to be higher than normal in homozygous but not in heterozygous DI rats. These are therefore unlikely to be the only factors responsible for the increased firing rates in the SO nucleus of DI rats. The results are consistent with the suggestion that the primary lesion in DI rats is an impairment of vasopressin biosynthesis which does not involve the electrical properties of the neurones.