Due to the limited treatment options for refractory hypotension and vascular hyporeactivity to vasoconstrictors, the new drug development is needed. The effect of the naturally-occurring flavonoids, oroxylin-A, wogonin and baicalein, in the presence of serum on the tensions of rat tail arteries was examined by using in vitro blood-vessel myography. We found that oroxylin-A (1-300 μM) dissolved in rat serum (OroA/RS), promoted RS-induced but endothelium-independent vasoconstrictions in a concentration-dependent manner. On the other hand, baicalein and wogonin (structurally close to OroA) did not affect or attenuated RS-induced constrictions in tail arterial rings. Repeated applications of OroA (300 μM)/RS induced reproducible and long-lasting constrictions without tachyphylaxis in the arterial rings. OroA/RS-induced vasoconstrictions were blocked by 5-HT2A receptor antagonist (ketanserin, 0.3 μM) and GRK2 antagonist (CMPD101, 10 μM), but not by the inhibitors of ET-1 receptor, AT-1 receptor, or EP1/2 receptor. These results suggest that OroA with its special structure may potentiate the endothelium-independent contractions induced by RS via the 5-HT/GRK2 pathway in tail arteries. The intervention of OroA may be beneficial for the acute management of endotoxemic hypotensive shock.