In rat striated muscle, serotonin (5-hydroxytryptamine; 5-HT) constricts large arterioles (first-order; A1) via 5-HT<sub>2</sub> receptor activation but dilates smaller arterioles via a 5-HT<sub>1</sub>-like receptor. In this preparation, A1 arterioles possess little basal tone. The purpose of this study was to determine if 5-HT would elicit A1 dilation if arteriolar tone was first induced. In anesthetized rats, A1 diameters of the cremaster muscle were measured via in vivo videomicroscopy. Topical application of angiotensin II caused a 26 ± 3 % constriction of the vessels. 5-HT dilated the preconstricted Als by 36 ± 7% while constricting normal tone Als by 33 ± 5%. This dilation was enhanced by blocking 5-HT<sub>2</sub> receptors with LY53857, but abolished with methysergide, a 5-HT<sub>1</sub> and 5-HT<sub>2</sub> receptor antagonist indicating that the dilation was mediated by a 5-HT<sub>1</sub>-like receptor. Thus, A1 arterioles possess both 5-HT<sub>2</sub> and 5-HT<sub>1</sub>-like receptors. The net result of 5-HT application in striated muscle, dilation or constriction, will depend on the initial tone of the vessels.