Three patients with autosomal-recessive nephrogenic diabetes insipidus (NDI), homozygous for mutations in the aquaporin 2 gene (AQP2), were tested for their fibrinolytic and hemodynamic responses to intravenous administration of 1-desamino-8-D-arginine vasopressin (DDAVP). They all showed an increase of tissue-type plasminogen activator antigen, facial flushing, an increase of heart rate and a decrease of diastolic blood pressure. These results confirm the hypothesis that NDI patients with an AQP2 defect can be discriminated from NDI patients with a vasopressin type 2 receptor defect by their normal extrarenal responses to DDAVP.