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Recent studies have found vasopressin to be antinatriuretic as well as antidiuretic. We therefore wished to determine the endogenous renal activity of vasopressin in spontaneously hypertensive rats (SHR) and one-kidney, one clip hypertensive (1K1C) rats.The renal effects of a vasopressin (V2) antagonist, [d(CH2)5,D-lle2,lle4]-arginine vasopressin (0, 10 or 30nmol/kg), were compared in SHR and 1K1C rats and their respective controls, the Wistar-Kyoto (WKY) rat and the Sprague-Dawley rat.The V2 antagonist produced a dose-related increase in urine flow rate and free water clearance in all groups studied (WKY rats, SHR, Sprague—Dawley rats and 1K1C rats). Sodium excretion and osmolar clearance were increased only in the WKY rats and 1K1C rats. The response in the SHR was significantly less than that observed in the WKY rats. At both doses of V2 antagonist investigated, the urine flow rate was two- to threefold greater in the WKY rats than in SHR, in spite of similar control levels. Conversely, the response oobserved in the acquired model of hypertension (1K1C rats) at the one dose of V2 antagonist investigated (30nmol/kg) was similar to that found in the control Sprague-Dawley rats, suggesting that the decreased response observed in SHR was not secondary to the increased blood pressure.These data demonstrate that the endogenous level of renal activity of vasopressin in SHR is suppressed compared with control WKY rats.