['The purpose of this study was to determine the influence of endothelium-released nitric oxide on uterine arterial tone and reactivity during pregnancy.', 'The effects of pregnancy on endothelial function were evaluated in isolated pressurized rat uterine arteries from late-pregnant rats (day 19 to 20) versus age-matched nonpregnant controls. The effects of nitric oxide synthase inhibition (N(omega)-nitro-L-arginine) on arterial tone and reactivity under basal and activated (phenylephrine) conditions were determined, as was arterial reactivity to endothelium-dependent (acetylcholine) and endothelium-independent (sodium nitroprusside) vasodilators, by evaluating changes in lumen diameter.', '(1) Maximal constriction to N(omega)-nitro-L-arginine was significantly enhanced under basal (nonstimulated) conditions in arteries from late-pregnant versus nonpregnant rats (changes in lumen diameter 37% +/- 8% vs 9.3 +/- 6.2%, respectively, p < 0.05). (2) Nitric oxide synthase blockade with 1 nmol/L N(omega)-nitro-L-arginine significantly increased phenylephrine sensitivity in arteries from late-pregnant animals (median effective concentration 115 +/- 23 nmol/L vs 33 +/- 8 nmol/L, control vs treated vessels, p < 0.05) but was without statistically significant effect on arteries from nonpregnant animals (control 255 +/- 164 nmol/L, treated 250 +/- 102 nmol/L, p > 0.05). (3) The threshold concentration of acetylcholine required to elicit endothelium-dependent dilation was significantly lower in late-pregnant versus nonpregnant arteries (1.4 +/- 0.2 nmol/L vs 12.2 +/- 3.8 nmol/L, p < 0.05). (4) Vascular smooth muscle sensitivity to an exogenous nitrodilator (sodium nitroprusside) was identical in late-pregnant versus nonpregnant vessels.'] Does gestation increase nitric oxide-mediated vasodilation in rat uterine arteries?