Introduction. Dimethylarginine dimethylaminohydrolase (DDAH) is a major endogenous regulatory enzyme controlling asymmetric dimethylarginine (ADMA) levels. ADMA, which inhibits nitric oxide synthase (NOS), is a mediator elevated in numerous disease states and several diseases associated with vascular dysfunction have been associated with aberrant levels of DDAH (Arrigoni et al., 2003; Palm et al., 2007). Investigating the endothelial cell (EC) function of individuals and assessing the nitric oxide (NO) pathway including the DDAH pathway, has been limited to invasive techniques and animal studies.  Recent developments in culture techniques to extract progenitive endothelial cells from whole blood, known as blood outgrowth endothelial cells (BOEC) or endothelial colony-forming cells (ECFC), means that endothelial cells can be isolated in a non-invasive manner from a relatively small amount of donor’s blood and represent an individual’s epigenetic makeup (Paschalaki et al., 2013). We therefore sought to measure DDAH and NOS mRNA in these cells to see if this was a viable model for assessing the NO pathway. Methods. The study was approved by the Kingston University Faculty Research Ethics Committee (Reference 1617/024).  Blood outgrowth endothelial cells (BOECs) were isolated and cultured from male and female adult healthy donors (n=5). BOECs were isolated as previously described until the formation of a characteristic cobbled-shaped morphology (Ormiston et al., 2015).  BOECs were confirmed to have classical EC surface marker expression (CD31, CD144, low CD34) using flow cytometry. Extracted BOECs and HUVECs (Sigma-Aldrich) were cultured under standard conditions and plated for 2-3 days, until confluent and treated under control or inflammatory conditions (TNF-α, 10ng/ml) for 24 hours.  RNA was extracted using TRIzol™ Reagent (Invitrogen) and cDNA synthesis performed using RevertAid First Strand cDNA Synthesis Kit (Thermo Scientific).  qRT-PCR was executed using primers for NOS2, NOS3, DDAH1 and DDAH2 mRNA, with ACTB and GAPDH used as reference genes.  Analysis used the 2–∆∆Ct method for quantification. Results. NOS3 expression was identical between HUVEC and BOEC and decreased following inflammatory stimulation to an equivalent level in both cell types (BOEC p=0.0275 and HUVEC p=0.0367, n=5). NOS2 was not detected under quiescent conditions or following inflammatory stimuli. DDAH2 mRNA levels were greater than DDAH1 in both BOEC and HUVEC (DDAH2>DDAH1 BOEC 15.2x p=0.0001, HUVEC 4.7x p=0.0248). Under quiescent and inflammatory conditions, DDAH1 gene expression did not differ between BOEC and HUVEC.  However, there was 2.9x more DDAH2 gene expression in BOEC than HUVEC at rest (p=0.0489, n=5) and following incubation with TNF-α, DDAH2 mRNA increased in HUVEC so that the two cell types expressed equivalent amounts.  Conclusions. This is the first time that DDAH expression has been examined in BOEC. BOEC express more DDAH2 than HUVEC under resting conditions with both cell types expressing more DDAH2 mRNA than DDAH1.  There was no difference in the expression of NOS3 in either EC type. DDAH2 is known to contribute to endothelial cell NO bioavailability, and as an endothelial cell model, these cells may provide better insight into an individual’s endothelial health than HUVEC. Keywords. BOEC; endothelial colony–forming cells; ECFC; DDAH; Nitric oxide; NO.