Studies on the vasopressor role of the antidiuretic hormone arginine-vasopressin (AVP) in DOC hypertension, in two-kidney Goldblatt hypertension, and in spontaneous hypertension of rats, and during acute blood pressure elevation after intracerebroventricular injection of angiotensin II and in glycerol-induced acute renal failure of rats are reviewed. For the measurement of plasma AVP a radioimmunoassay has been developed. For this assay, a series of criteria has been met which allows the conclusion that, in plasma of rats, the antibody measures AVP only. For the blockade of vasopressor effects of AVP a specific antiserum has been used. On the basis of a series of control studies it has been concluded, but not proven that the antiserum lowers blood pressure exclusively by blockade of AVP. It could be shown that in the various animal models of hypertension and of acute blood pressure elevation AVP exerts systemic vasoconstriction when its plasma concentrations are elevated. In those models where the renin-angiotensin system played no role in blood pressure control, the height of blood pressure was closely related to the plasma AVP concentrations. When this relationship was compared with that obtained after the i.v. infusion or injection of AVP, a marked shift to the left became apparent. Hence, sensitization to the vasopressor effect of AVP had occurred, the factor of sensitization amounting to more than 1,000. It is concluded that AVP is not only an antidiuretic hormone but also a vasopressor hormone, and that any systemic vasopressor effect of AVP requires a mechanism of sensitization.