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Background: Increased vasoreactivity due to reduced endothelial NO bioavailability is an underlying feature of cardiovascular disease, including hypertension. In small resistance arteries, declining NO enhances vascular smooth muscle (VSM) reactivity partly by enabling rapid depolarizing Ca 2+ -based spikes that underlie vasospasm. The endogenous NO synthase inhibitor asymmetric dimethylarginine (ADMA) is metabolized by DDAH1 (dimethylarginine dimethylaminohydrolase 1) and elevated in cardiovascular disease. We hypothesized ADMA might enable VSM spikes and vasospasm by reducing NO bioavailability, which is opposed by DDAH1 activity and L-arginine., Methods: Rat isolated small mesenteric arteries and myogenic rat-isolated intraseptal coronary arteries (RCA) were studied using myography, VSM intracellular recording, Ca 2+ imaging, and DDAH1 immunolabeling. Exogenous ADMA was used to inhibit NO synthase and a selective DDAH1 inhibitor, N G -(2-methoxyethyl) arginine, to assess the functional impact of ADMA metabolism., Results: ADMA enhanced rat-isolated small mesenteric arteries vasoreactivity to the α 1 -adrenoceptor agonist, phenylephrine by enabling T-type voltage-gated calcium channel-dependent depolarizing spikes. However, some endothelium-dependent NO-vasorelaxation remained, which was sensitive to DDAH1-inhibition with N G -(2-methoxyethyl) arginine. In myogenically active RCA, ADMA alone stimulated depolarizing Ca 2+ spikes and marked vasoconstriction, while NO vasorelaxation was abolished. DDAH1 expression was greater in rat-isolated small mesenteric arteries endothelium compared with RCA, but low in VSM of both arteries. L-arginine prevented depolarizing spikes and protected NO-vasorelaxation in rat-isolated small mesenteric artery and RCA., Conclusions: ADMA increases VSM electrical excitability enhancing vasoreactivity. Endothelial DDAH1 reduces this effect, and low levels of DDAH1 in RCAs may render them susceptible to endothelial dysfunction contributing to vasospasm, changes opposed by L-arginine., Competing Interests: None.