This study tested the hypothesis that afferent arteriolar responses to purinoceptor activation are attenuated, and Ca2+ signaling mechanisms are responsible for the blunted preglomerular vascular reactivity in angiotensin II (Ang II) hypertension. Experiments determined the effects of ATP, the P2X1 agonist β,γ-methylene ATP or the P2Y agonist UTP on arteriolar diameter using the juxtamedullary nephron technique and on renal myocyte intracellular Ca2+ concentration ([Ca2+] i) using single cell fluorescence microscopy. Six or 13 days of Ang II infusion significantly attenuated the vasoconstrictor responses to ATP and β,γ-methylene ATP (P<0.05). During exposure to ATP (1, 10, and 100 μmol/L), afferent diameter declined by 17±2%, 29±3%, and 30±2% in normal control rats and 8±3%, 7±3%, and 22±3% in kidneys of Ang II-infused rats (13 days). Renal myocyte intracellular calcium responses to ATP or β,γ-methylene ATP were also decreased in Ang II hypertensive rats. In myocytes of control rats, peak increases in [Ca2+]i averaged 107±21, 170±38, and 478±79 nmol/L at ATP concentrations of 1, 10, and 100 μmol/L, respectively. Ang II infusion for 13 days decreased the peak responses to ATP (1, 10, and 100 μmol/L) to 65±13, 102±20, and 367±73 nmol/L, respectively. The peak increases in [Ca2+] i in response to β,γ-methylene ATP were also reduced in Ang II hypertensive rats. However, angiotensin hypertension did not change the UTP-mediated vasoconstrictor responses or the myocyte calcium responses to UTP. These results indicate that the impaired autoregulatory response observed in Ang II-dependent hypertension can be attributed to impairment of P2X, receptor-mediated signal transduction. © 2005 American Heart Association, Inc.