Experiments were designed to compare basal and acetylcholine-induced release of endothelium-derived relaxing factor at different flow rates in the aorta of the normotensive (Wistar-Kyoto; WKY) and spontaneously hypertensive (SHR) rat. Aortic segments (with endothelium) of either WKY or SHR were perfused at different flow rates (1 or 4 mL/min) with modified Krebs-Ringer solution; the relaxing activity of the perfusate from the two types of segments was bioassayed by measuring the isometric force in rings (without endothelium, and contracted with norepinephrine) of aortas of both WKY and SHR. All studies were performed in the presence of indomethacin to prevent endothelial production of prostacyclin and other vasodilator prostanoids. The basal release of endothelium-derived relaxing factor was not significantly affected by the flow rate in either the WKY or the SHR aortas. At the two flow rates, and with both types of bioassay rings, the basal release of endothelium-derived relaxing factor was smaller with SHR than with WKY aortas, but this reached significance only at 4 mL/min using the SHR-aorta as bioassay tissue. Both with WKY and SHR aortas the release of endothelium-derived relaxing factor evoked by acetylcholine was significantly larger at 1 than at 4 mL/min; no significant differences in responsiveness to acetylcholine were noted between WKY and SHR segments. There was also no difference in responsiveness of WKY and SHR bioassay rings to acetylcholine and acetylcholine-induced release of endothelium-derived relaxing factor. These experiments suggest that prolonged exposure to increases in shear stress reduces the ability of the endothelium to release endothelium-derived relaxing factor in responses to muscarinic activation.