Dynamics of AVP secretion in a 14-year-old girl with essential hypernatremia, psychomotor retardation and optic nerve coloboma, are reported. Basal levels of AVP were similar to those of a control population, but disproportionately low in relation with natremies. Hypertonic saline and hydric restriction did not alter the AVP basal values, which were, instead, stimulated with orthostatism. AVP decreased during a water surcharge, but delayed elimination of water was observed. The existence of a moderate volume deficit, not corrigible with a chronic surcharge of water, together with the reversed diurnal pattern of water excretion and the AVP data, suggest--as a physiopathological basis of the syndrome--a severe anomaly of the osmoreceptors, with alteration of thirst and of the osmodependent AVP responses, so that the AVP secretion was regulated exclusively through volumetric mechanisms.