The electrophysiological effects of D-myo-inositol 1,3,4,5,6-pentakisphosphate (InsP₅) and D-myo-inositol hexakisphosphate (InsP₆), which represent the main cellular inositol polyphosphates, were studied on L-type Ca²⁺ channels in single myocytes of rat portal vein. Intracellular infusion of InsP₅ (up to 50 µM) or 10 µM InsP₆ had no action on Ba²⁺ current, whereas 50 µM InsP₆ or 10 µM InsP₅ plus 10 µM InsP₆ (InsP_5,6) stimulated the inward current. The stimulatory effect of InsP_5,6 was also obtained in external Ca²⁺-containing solution. The stimulated Ba²⁺ current retained the properties of L-type Ba²⁺ current and was oxodipine sensitive. PKC inhibitors Ro 32-0432 (up to 500 nM), GF109203X (5 µM) or calphostin C (100 nM) abolished the InsP_5,6-induced stimulation. Neither the PKA inhibitor H89 (1 µM) nor the protein phosphatase inhibitors okadaic acid (500 nM) or cypermethrin (1 µM) prevented or mimicked the InsP_5,6-induced stimulation of Ba²⁺ current. However, InsP₅ or InsP₆ could mimick some effects of protein phosphatase inhibitor so as to extend after washing-out forskolin the stimulatory effects of the adenylyl cyclase activator on Ba²⁺ current. These results indicate that InsP₅ and InsP₆ may act as intracellular messengers in modulating L-type Ca²⁺ channel activity and so could be implicated in mediator-induced contractions of vascular smooth muscle cells.