We examined the interrelationships between the pressor response to the administration of norepinephrine and arginine vasopressin and baroreflex function in rats with hypertension of two days' duration induced by heminephrectomy and a clip placed on the right renal artery (2-day clipped rats). Mean arterial pressure was higher in the 2-day clipped rats than in heminephrectomized rats without clips (sham-operated rats). The pressor response in the 2-day clipped rats to both agents increased as compared to the sham-operated rats. This hyperresponsiveness was attenuated by administering an angiotensin II antagonist, [1-Sar, 8-Ile] angiotensin II. Baroreflex sensitivity was studied by measuring changes in arterial pressure and pulse interval in response to the injection of phenylephrine. Baroreflex sensitivity was not decreased but markedly increased in the 2-day clipped rats and unaffected by infusing the angiotensin II antagonist. These results provide evidence that 1) in the 2-day clipped rats there are exaggerated pressor responses to vasoconstrictors; 2) the hyperresponsiveness is not causally related to the change of baroreflex sensitivity; and 3) angiotensin II plays a significant role in the increased pressor responses; however, the baroreflex mechanism is not involved in attenuation of the hyperresponsiveness by the angiotensin II antagonist.