The present study was conducted to determine the contribution of nitric oxide to angiotensin II (Ang II) reactivity of afferent and efferent arterioles from Ang II-infused hypertensive rats. Experiments were performed in vitro with the blood-perfused juxtamedullary nephron technique in kidneys harvested from hypertensive Sprague-Dawley rats (181+/-1 mm Hg) that had received 60 ng/min Ang II subcutaneously for 13 days. Superfusion with 0.1, 1, and 10 nmol/L Ang II reduced afferent arteriolar diameter (18.1+/-0.6 microm; n=12) by 10.0+/-0.7%, 28.1+/-1.7%, and 52.8+/-1.9%, respectively, and efferent arteriolar diameter (17.2+/-1.4 microm; n=8) decreased by 9.3+/-0.7%, 27.0+/-1.2%, and 50.4+/-1.6%, respectively. Nitric oxide synthase inhibition with 100 micromol/L N(omega)-nitro-L-arginine (NLA) reduced resting afferent and efferent arteriolar diameters to 14.7+/-0.4 and 14.3+/-1.2 microm, respectively, and enhanced afferent but not efferent arteriolar reactivity to Ang II. The enhanced afferent arteriolar reactivity to Ang II was eliminated by addition of the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP, 10 micromol/L), which reversed the NLA-induced decrease in diameter. Addition of 10 micromol/L SNAP, without NLA, blunted efferent but not afferent arteriolar reactivity to Ang II. Afferent (n=7) and efferent arteriolar diameters (n=6) decreased by 48.5+/-2.2% and 41.0+/-1.9%, respectively, in response to 10 nmol/L Ang II. These results suggest that in this model of hypertension, maintained nitric oxide production in afferent arterioles counteracts the enhanced afferent arteriolar reactivity that occurs in Ang II-induced hypertension.