It was hypothesized that NaF induces calcium sensitization in Ca2＋-controlled solution in permeabilized rat mesenteric arteries. Rat mesenteric arteries were permeabilized with Ղ- escin and subjected to tension measurement. NaF potentiated the concentration-response curves to Ca2＋ (decreased EC50 and increased Emax). Cumulative addition of NaF (4.0, 8.0 and 16 mM) also increased vascular tension in Ca2＋-controlled solution at pCa 7.0 or pCa 6.5, but not at pCa 8.0. NaF-induced vasocontraction and GTPՃS-induced vasocontraction were not additive. NaF-induced vasocontraction at pCa 7.0 was inhibited by pretreatment with Rho kinase inhibitors H1152 or Y27632 but not with a MLCK inhibitor ML-7 or a PKC inhibitor Ro31-8220. NaF induces calcium sensitization in a Ca2＋- dependent manner in Ղ-escin-permeabilized rat mesenteric arteries. These results suggest that NaF is an activator of the Rho kinase signaling pathway during vascular contraction.