The action of endothelin in small intestinal resistance vessels of the guinea pig was studied by examining submucosal arteriole vasoactivity in vitro and electrical properties of mesenteric arteriole smooth muscle cells. Endothelin-1 (ET-1) constricted submucosal arterioles with a half-maximal effective concentration of 170 pM. ET-3 caused detectable constriction with a minimum of 20 nM. The ET-1 response was prolonged, with a time to 90% relaxation of 41 ± 2.8 min after washout. The ET_A antagonist BQ-123 (200 nM) decreased the sensitivity to ET-1 ~40-fold. Arterioles preconstricted with prostaglandin F₂ did not relax when superfused with ET-1, ET-3, or an ET_B agonist, IRL-1620, and pretreatment with the nitric oxide synthase inhibitor N^G-monomethyl-L-arginine was ineffective in countering ET-1-induced constriction, indicating the absence of functional ET_B receptors. Resting membrane potential in isolated cells was characterized by transient hyperpolarizing spikes (THs). ET-1 (20 nM) increased TH frequency and caused the emergence of a larger amplitude population. Under voltage clamp, spontaneous transient outward currents (STOCs) were seen that reversed at the K⁺ equilibrium potential. ET-1 increased STOC frequency and amplitude. Iberiotoxin (IBTX; 200 nM), a maxi-K⁺ channel antagonist, blocked the ET-1-induced THs and reduced STOC activity. IBTX or tetraethylammonium increased the rate and extent of ET-1-induced arteriole constriction. We suggest that ET-1-induced vasoactivity of ileal resistance arterioles involves ET_A receptor-mediated early activation of maxi-K⁺ channels that serves to counter strong constriction.