Variation in anatomic extent of the collateral circulation is an important determinant of variation in the severity of ischemic stroke and efficacy of revascularization therapies. Yet pial collateral number and lumen diameter decrease with age, at least in mice. It is not known if this can be mitigated. We tested whether exercise training can accomplish this and if it also affects diameter of the posterior communicating collalterals (PComs) and primary cerebral arteries (ICA, BA, MCA, ACA, PCA). We randomized 30 male and 30 female, 12 months-old (~40 human years, hy) C57BL/6J mice to either sedentary or voluntary wheel-running (daily distance run was measured). At 25 mos-age (~70 hy), permanent MCA occlusion was followed 24h later by vascular casting after maximal dilation and by determination of infarct volume. Controls for aging were 3 mos-old sedentary mice (~16 hy). Training effect was confirmed by muscle fiber-type switching, body weight and cardiac hypertrophy (all p<0.05). Exercise prevented age-associated loss of collateral number and diameter (p=0.049 and 0.005, n=13-14) and reduced infarct volume by 50%, ie, to that seen in 3 months-old mice (p=0.01, n=7). Unlike pial collaterals, PCom diameter and number (ie, present bilaterally, unilaterally or absent) were unaffected by exercise. Of further interest, diameter of the primary cerebral arteries increased with aging alone (119 ± 1 vs 134 ± 5 μm averaged combined diameter, p=0.02, n=10-14); this effect tended to increase further with exercise (143 ± 4 μm, p=0.07, n=13). Mechanistically, exercise increased vascular expression (assessed by immunohisto-chemistry) of total eNOS (p=0.03, n=5-6), phospho-eNOS (p=0.004, n=5) and a marker of anti-oxidative stress (SOD, p=0.008, n=5-6; but not HO-1, p=0.40, n=6). It did not alter a marker of aging (p16, p=0.42, n=5); analysis of additional targets is underway. In conclusion, exercise training prevented age-induced rarefaction of pial collaterals and reduced infarct volume. In addition and unexpectedly, aging also caused outward remodeling of the primary cerebral arteries, and exercise training tended to further augment this. These benefits of regular aerobic exercise were associated with increased eNOS bioavailability and reduced oxidative stress.