Although epoxyeicosatrienoic acids, cytochrome P-450 mono-oxygenase metabolites of arachidonic acid, have been demonstrated to play a crucial role in endothelial cell Ca2+ homeostasis and endothelium-dependent vasorelaxation, the understanding of the actions of epoxyeicosatrienoic acids is limited. In this study, the effect of epoxyeicosatrienoic acids on tyrosine kinase in endothelial cell homogenate was investigated. 11,12-Epoxyeicosatrienoic acid increased tyrosine kinase activity in a concentration dependent manner (EC50 = 11.7 nM). Arachidonic acid in much higher concentrations (20 microM) mimicked the effect of the epoxyeicosatrienoic acid on tyrosine kinase. This effect of arachidonic acid was abolished in the presence of the cytochrome P-450 mono-oxygenase inhibitor thiopentone sodium, indicating that arachidonic acids needs to be converted to epoxyeicosatrienoic acids by the endothelial cytochrome P-450 mono-oxygenase to stimulate tyrosine kinase. These data describe a novel aspect of the actions of epoxyeicosatrienoic acids, and show that in addition to K+ channel activation, epoxyeicosatrienoic acids also regulate tyrosine kinase activated signaling pathways in endothelial cell activation.