A study was carried out how the sexual difference influences the increase in blood pressure (BP) induced by arginine vasopressin (AVP), and how the binding characteristics of 3H-labelled AVP on membranes prepared from the vascular bed were affected. After the administration of various doses of AVP, a significantly higher BP increase was observed in male rats than in females. The vasopressor effect of AVP was reduced in males following orchidectomy or administration of the antiandrogen cyproterone acetate. The vasopressin (VP) antagonist d(CH2)5Tyr(Me)AVP diminished the BP response to AVP in both sexes. The plasma AVP level was found to be much higher in males than in females, but it was decreased to the level of females after orchidectomy. The density of AVP-binding sites in the aorta membrane preparation was smaller in females, and in orchidectomized or cyproterone acetate-treated male rats than in the control males. The results demonstrate that testosterone upregulates the number of AVP-binding sites, leading to an increase in the pressor response to AVP in the rat vascular bed.