Interleukin (IL) 1 and IL-6 have been implicated in the decreased systemic vascular resistance of septic shock; however, their sites of action and underlying mechanisms remain unclear. This study determined the effects of IL-1 and IL-6 on rat skeletal muscle arterioles using both in vitro and in vivo preparations. In the in vitro preparation, first order cremasteric arterioles were isolated from rats, cannulated with micropipettes, pressurized to 70 mmHg, superfused with physiologic saline solution, and allowed to achieve spontaneous basal tone in the absence of intraluminal flow. In the in vivo preparation, the cremaster muscle of anesthetized rats was surgically opened, secured as a flat sheet over an optical pedestal, and superfused with physiologic saline solution. Responses of third order arterioles were studied using transillumination video microscopy. In both arteriolar preparations, vessel diameter and phenylephrine (PE) responsiveness were assessed before and after cytokine exposure and washout. In vitro exposure of IL-1 (20 ng/mL (n=8) or 60 ng/mL (n=4)) or IL-6 (500 U/mL (n=2) or 1,000 U/mL (n=4)) for 1 h did not cause arteriolar vasodilation or change in PE responsiveness. However, during a 1 h in vivo exposure of IL-1 (.01, .1, 1.0, or 20 ng/mL), arteriolar diameter increased from 47+/-2 to 58+/-2% of maximum diameter (Dmax) (n=14, p < .0001), from 45+/-2 to 69+/-3% of Dmax (n=14, p < .0001), from 45+/-3 to 96+/-2% of Dmax (n=8, p < .0001), and from 47+/-4 to 96+/-1% of Dmax (n=14, p < .0001), respectively. Cytokine washout resulted in arteriolar return to basal diameter. Cytokine exposure and washout did not affect arteriolar PE responsiveness. In vivo exposure of IL-6 (10, 50, or 250 U/mL) for 1 h increased diameter from 47+/-2 to 57+/-2% of Dmax (n=14, p < .0001), from 46+/-2 to 75+/-3% of Dmax (n=14, p < .0001), and from 46+/-2 to 68+/-4% of Dmax (n=15, p < .0001), respectively. After washout of IL-6 (10, 50, or 250 U/mL), arteriolar dilation persisted, from 16.3+/-.5 to 20.1+/-1.4 microm (n=14, p < .003), from 16.1+/-.4 to 20.1+/-.6 microm (n=14, p < .0001), and from 17.7+/-.4 to 22.5+/-.8 microm (n=15, p < .0001), respectively. There was no change in PE responsiveness. These data indicate that the cytokines IL-1 and IL-6 are potent dilating agents for skeletal muscle resistance vessels under in vivo conditions. However, given that IL-1 and IL-6 are ineffective in causing relaxation of similar arterioles under isolated in vitro conditions, it is suggested that IL-1 and IL-6 interact with parenchymal or intravascular factors to elicit arteriolar relaxation.