The possible mechanism is involved in the effects ofSpirulina platensison vascular reactivity. Animals were divided into sedentary group (SG) and sedentary groups supplemented withS. platensisat doses of 50 (SG50), 150 (SG150), and 500 mg/kg (SG500). To evaluate reactivity, cumulative concentration-response curves were constructed for phenylephrine and acetylcholine. To evaluate the involvement of the nitric oxide (NO) pathway, aorta tissue was preincubated with L-NAME and a new curve was then obtained for phenylephrine. Biochemical analyses were performed to evaluate nitrite levels, lipid peroxidation, and antioxidant activity. To contractile reactivity, only SG500 (pD2=5.6±0.04vs.6.1±0.06,6.2±0.02, and6.2±0.04) showed reduction in phenylephrine contractile potency. L-NAME caused a higher contractile response to phenylephrine in SG150 and SG500. To relaxation, curves for SG150 (pD2=7.0±0.08vs.6.4±0.06) and SG500 (pD2=7.3±0.02vs.6.4±0.06) were shifted to the left, more so in SG500. Nitrite was increased in SG150 and SG500. Lipid peroxidation was reduced, and oxidation inhibition was increased in all supplemented groups, indicating enhanced antioxidant activity. Chronic supplementation withS. platensis(150/500 mg/kg) caused a decrease in contractile response and increase in relaxation and nitrite levels, indicating greater NO production, due to decreased oxidative stress and increased antioxidant activity.