The present study evaluated the acute effects of ANG II (5-480 ng/kg iv) and phenylephrine (PE; 0.2-146 µg/kg iv) on total renal (RBF) and medullary blood flow (MBF) in anesthetized Lyon hypertensive (LH) and low-blood-pressure (LL) rats. ANG II and PE induced dose-dependent decreases in both RBF and MBF, which were greater in LH than in LL rats. Interestingly, after ANG II, but not after PE, the initial medullary vasoconstriction was followed by a long-lasting and dose-dependent vasodilation that was significantly blunted in LH compared with LL rats. The mechanisms of the MBF effects of ANG II were studied in LL rats only. Blockade of AT₁ receptors with losartan (10 mg/kg) abolished all the effects of ANG II, whereas AT₂ receptor blockade with PD-123319 (50 µg · kg¹ · min¹ iv) did not change these effects. Indomethacin (5 mg/kg) decreased by ~90% the medullary vasodilation induced by the lowest doses of ANG II (from 15 ng/kg). In contrast, N^G-nitro-L-arginine methyl ester (10 mg/kg and 0.1 mg · kg¹ · min¹ iv) and the bradykinin B₂-receptor antagonist HOE-140 (20 µg/kg and 10 µg · kg¹ · min¹ iv) markedly lowered the medullary vasodilation at the highest doses of ANG II only. In conclusion, this study shows that LH rats exhibit an altered MBF response to ANG II compared with LL rats and indicates that the AT₁ receptor-mediated medullary vasodilator response to low doses of ANG II is mainly due to the release of PGs, whereas the dilator response to high doses of ANG II has additional nitric oxide- and kinin-dependent components.