The effect on water metabolism of 1-deamino-8-D-arginine vasopressin and lysine vasopressin have been studied and compared in 20 vasopressin-sensitive and 2 ADH-resistant diabetes insipidus patients. In every case of ADH-sensitive diabetes insipidus, diuresis decreased and the urinary osmolality increased more markedly and for a longer time with the former than with the latter drug. Both drugs were ineffective in patients with ADH-resistant diabetes insipidus. Administration of 1-deamino-8-D-arginine vasopressin did not cause any side effect. It is concluded that 1-deamino-8-D-arginine vasopressin can successfully be employed in the treatment of ADH-sensitive diabetes insipidus.