Introduction: An association exists between elevated levels of the endogenous nitric oxide synthase (NOS) inhibitor asymmetric dimethylarginine (ADMA) and a number of cardiovascular diseases. However, to date, no studies have shown a causal relationship between elevated ADMA and cardiovascular pathology. Dimethylarginine dimethylaminohydrolase (DDAH) catalyses the hydrolysis of ADMA in vivo. In order to test the hypothesis that elevated ADMA levels are causal in the pathogenesis of cardiovascular disease we generated mice deficient in DDAH1 and studied their systemic and pulmonary circulations both in vivo and ex vivo. Methods: Mice were generated by targeted deletion of exon 1 of the DDAH1 gene. Homozygous deletion of DDAH1 was lethal in utero whilst heterozygous DDAH1 haploinsufficient mice were viable. DDAH1 protein, DDAH activity and ADMA levels in tissue and plasma were measured in both DDAH1 +/- mice and their wild type littermates. The vascular reactivity of isolated aortas and pulmonary arteries and the effects of 1mM L-arginine on acetylcholine induced relaxations were also assessed. In addition to histologicical examination of the pulmonary vasculature. In vivo studies were conducted in minimally anesthetized DDAH1+/- and wild type littermates. Mean arterial and right ventricular pressures were measured using a fluid filled catheter inserted into the right carotid artery or right ventricle respectively and cardiac output, heart rate and systemic vascular resistance were simultaneously measured using echocardiography. Results: Heterozygous deletion of DDAH1 resulted in reduced DDAH1 protein and activity and elevated tissue and plasma ADMA levels. Endothelial dysfunction was observed in both systemic and pulmonary vessels from DDAH1 +/- mice ex vivo an effect that was fully reversible by L-arginine. In vivo, mean arterial blood pressure and systemic vascular resistance were increased whilst cardiac output and heart rate were decreased. Furthermore increased right ventricular pressures together with increased muscularisation of resistance arteries in the pulmonary vasculature was observed. Conclusion: This study has demonstrated that in DDAH1 +/- mice, elevated ADMA causes endothelial dysfunction and results in increases in both mean arterial and pulmonary pressures indicating that elevated ADMA may be causal in the pathogenesis of cardiovascular disease.