1. The effect of α-adrenoceptor blockade by phentolamine on plasma arginine vasopressin, on vasopressin dependency of blood pressure and on responsiveness to exogenous lysine vasopressin was studied in conscious normotensive and in deoxycorticosterone (DOC)/salt-treated hypertensive rats.

2. Phentolamine infusion reduced blood pressure of DOC-treated hypertensive rats to levels significantly lower than those of similarly treated Wistar control animals.

3. Plasma vasopressin levels of the DOC-treated hypertensive and normotensive control rats were similar. Phentolamine infusion increased plasma arginine vasopressin levels to 109.4 ±21.8 pg/ml in the DOC-treated rats and to 44.4 ± 8.2 pg/ml in the control animals. It also enhanced the pressor response to exogenous lysine vasopressin in both groups.

4. Administration of the vasopressin analogue dPVDAVP had no effect on blood pressure in either the DOC-treated or the control group but induced a similar fall in blood pressure in both groups during α-adrenoceptor blockade with phentolamine.

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