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 [micro sign]m; 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 [micro sign]m; n=8) decreased by 9.3 +/- 0.7%, 27.0 +/- 1.2%, and 50.4 +/- 1.6%, respectively. Nitric oxide synthase inhibition with 100 [micro sign]mol/L Nomega-nitro-L-arginine(NLA) reduced resting afferent and efferent arteriolar diameters to 14.7 +/- 0.4 and 14.3 +/- 1.2 [micro sign]m, 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 [micro sign]mol/L), which reversed the NLA-induced decrease in diameter. Addition of 10 [micro sign]mol/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. (Hypertension. 1998;31:1255-1260.)