Previous studies have established the hepatoprotective, gastroprotective, hypolipidemic and hypoglycemic effects of kolaviron (KV), a biflavonoid complex from Garcinia kola seeds. In this study, we investigated the mechanisms involved in the vasorelaxant effects of KV in isolated superior mesenteric arteries from normotensive rats. KV (1, 10, 30, 100, 300, 500 and 1,000 μg/ml) concentration-dependently inhibited the contractions induced by phenylephrine (PHE) (10 μM) and KCl (80 mM) in both endothelium-intact (E_max = 58.3 ± 1.7% and 51.4 ± 1.3%, respectively) and -denuded rings (E_max = 59.3 ± 5.5% and 64.3 ± 2.4%, respectively). Furthermore, KV reduced CaCl₂-induced contraction in Ca²⁺-free medium containing KCl 60 mM, thus acting as a Ca²⁺-antagonist. In addition, KV inhibited the transient contraction by PHE in Ca²⁺-free medium containing EGTA, suggesting a possible action on the release of intracellular Ca²⁺ via the inositol-1,4,5-triphosphate (IP₃) pathway. KV is not a specific α-adrenoceptor blocker, since it also caused a concentration-dependent inhibition of contractile responses to KCl, suggesting that KV also blocks the L-type Ca²⁺-channel. As a Ca²⁺ antagonist, KV (100 μg/ml) potentiates the relaxant effects of nifedipine in denuded rings (E_max = 97.6 ± 1.2%; control = 75.1 ± 3.0%, P<0.05). Also, the vasorelaxation induced by KV was significantly inhibited after pre-treatment of the denuded rings with 4-aminopyridine (4-AP) 1 mM, a selective blocker of voltage-dependent K⁺ (K_v) channels and, tetraethylammonium (TEA) 1 mM or charybdotoxin (ChTX) 0.1 μM, non-selective blockers of large and intermediate conductance Ca²⁺-activated K⁺ (BK_Ca) channels. In contrast, neither glibenclamide (10 μM), BaCl2 (1 mM) nor apamin (0.1 μM), blockers of K_ATP, K_IR and SK_Ca channels, respectively affected the KV-induced vasorelaxation. In conclusion, our results provide functional evidence that the vasorelaxant effects by KV involve extracellular Ca²⁺ influx blockade, inhibition of intracellular Ca²⁺ release and the opening of K⁺ channels sensitive to 4-AP and ChTX with a resultant membrane hyperpolarization/ repolarization.