The aims of this study were to investigate, for the first time, the effects of endothelial factor inhibition on both the magnitude and dynamics of the response of isolated small coronary arteries to intraluminal flow. Isolated rat coronary arteries were mounted on a pressure myograph and left to develop myogenic tone. Flow was introduced and maintained until stable diameters were attained. Dilatory responses were observed which were maximal at low flow rates (5–10 µl/min) and thus shear stresses (1–2 dyn/cm<sup>2</sup>). These responses were transient in nature. Transient dilations were also observed upon cessation of flow. All responses (to 5 µl/min) were endothelium dependent and were completely abolished by addition of charybdotoxin (100 n M) and apamin (100–500 n M) suggesting an important role for a hyperpolarizing mechanism most likely involving an endothelium-derived hyperpolarizing factor. However, inhibitors of nitric oxide synthase (L-NNA; 100 µ M) or cyclo-oxygenase (indomethacin; 10 µ M) also modulated the response causing an increase and decrease in maximum vasodilation, respectively. By examining the time course we showed that both agents also made the response significantly more transient in nature. These results show that inhibition of endothelial factor pathways can influence both the magnitude and dynamics of the response of isolated rat coronary arteries to flow.