Abstract Objectives Our laboratory previously reported that docosahexaenoic acid (DHA) activates p38 mitogen-activated protein kinase (MAPK) differently in growing and quiescent human endothelial cells, which represent the atherogenic and healthy states in vivo, respectively. Endothelial nitric oxide synthase (eNOS) activity differs between these two states. Since eNOS can be regulated by p38MAPK and mitogen-stimulated kinase (MSK) is a p38MAPK substrate involved in cell proliferation and inflammation, we hypothesized that DHA's atheroprotective actions require eNOS activation via the p38MAPK/MSK pathway. Thus, our objective was to investigate the role of p38MAPK/MSK in the eNOS response to DHA and determine whether the proposed pathway is growth state-sensitive. Methods EA.hy926 cells were cultured on Matrigel-coated plates to sub-confluent and quiescent states and treated with DHA ± SB202190 or SB747651A, inhibitors of p38MAPK and MSK, respectively. eNOS activation was quantified by Western blot detection of Ser1177 phosphorylation. Results eNOS activation by DHA in EA.hy926 cells was concentration-, time-, and growth state-dependent. p38MAPK inhibition suppressed eNOS activity in sub-confluent cells and increased eNOS activity in quiescent cells, while MSK inhibition suppressed eNOS activity in both growth states. eNOS activity remained suppressed with DHA treatment under MSK inhibition and showed no dose- or growth state-dependent effects. In contrast, when p38MAPK was inhibited, high dose DHA activated eNOS in sub-confluent cells, but dose-dependently decreased the elevated eNOS activity of quiescent cells. Conclusions eNOS activity in endothelial cells is modulated by DHA via p38MAPK and MSK. The growth state-dependent regulation of p38MAPK and eNOS by DHA provides novel insight into the molecular mechanisms of DHA's atheroprotective actions in relation to health status. Funding Sources Research Manitoba, St Boniface Hospital Foundation, University of Manitoba - GETS