We aimed to test the hypothesis that as a consequence of short-term daily exercise, flow (shear stress)-dependent dilation and its mediation by the endothelium are altered in skeletal muscle arterioles. After initial familiarization with the protocol, rats ran on a treadmill once a day (with gradually increasing intensity up to 40 minutes and 28 m/min) for approximately 3 weeks (EX group); a control group remained sedentary (SED group). The active (internal) diameters of isolated gracilis muscle arterioles of SED and EX rats at 80 mm Hg were significantly different (55.2 +/- 2.1 and 49.3 +/- 2.0 microns, P < .05), and their passive diameters (in Ca(2+)-free solution) were 105.3 +/- 3.1 and 111.2 +/- 2.4 microns (not significantly different), respectively. Increases in flow of the perfusion solution from 0 to 12 microL/min elicited a significantly greater increase in diameter of EX arterioles (by 83.5% at maximum flow). This enhanced sensitivity maintained a lower shear stress in EX arterioles (15 to 20 dyne/cm2) compared with SED arterioles (25 to 35 dyne/cm2). In both SED and EX arterioles, flow-dependent dilation was eliminated after removal of the endothelium. Either N omega-nitro-L-arginine, a nitric oxide synthase inhibitor, or indomethacin, an inhibitor of prostaglandin synthesis, shifted the flow-diameter and calculated wall shear stress-diameter curves significantly to the right. Each of the inhibitors reduced flow-dependent dilation to a similar degree (approximately 40% to 45%); their combined administration nearly completely eliminated the dilation of arterioles of both SED and EX rats.(ABSTRACT TRUNCATED AT 250 WORDS)