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Three contrast media (mcglumine diatrizoate, meglumine iothalamate and sodium diatrizoate) produced a characteristic biphasic renal hemodynamic response in dogs. An initial transient vasodilatation, which is the common response to contrast media of all vascular beds, was followed by a more prolonged period of vasoconstriction, which is unique to the kidney. Phenoxybenzamine blocked the renal vasoconstriction produced by norepinephrine and 5–hydroxytryptamine but did not influence the biphasic response to the contrast media. The vasoconstriction caused by a bolus of angiotensin or by the contrast media was completely abolished during angiotensin tachyphylaxis, however. Thus, neither norepinephrine nor 5–hydroxytryptamine was involved in mediating the vasoconstriction, whereas angiotensin was implicated. The vasoconstrictor response to contrast media apparently is related to the hyper tonicity of these agents, activation of the renin-angiotensin system and the local conversion of angiotensin I to angiotensin II within the kidney.