The most active component in smooth muscle contraction, isolated from the whole venom of the marine snail Conus tessulatus, has a molecular mass of about 55 kDa. The toxin protein, tessulatus toxin, appeared to be constituted by two distinct polypeptide bands of 26 kDa and 29 kDa. The pure toxin caused a marked contraction of both guinea-pig ileum and rabbit aorta at nanomolar concentrations. Tessulatus-toxin-induced contraction was indirectly prevented by classical inhibitors of the voltage-dependent Ca2+ channel. Tessulatus toxin caused a large increase in the initial rate of 45Ca2+ uptake by cardiac cells. This uptake was insensitive to Ca2+ channel blockers at concentrations 100-1000 fold higher than those known to block voltage-dependent Ca2+ channels in these cells. Voltage clamp experiments have confirmed that tessulatus toxin was not directly active on the Ca2+ current. Tessulatus-toxin-stimulated 45Ca2+ influx was inhibited by dichlorobenzamil and suppressed when Na+ was substituted by Li+, indicating that the toxin acted via activation of the Na+/Ca2+ exchange system in cardiac cells. Activation by tessulatus toxin of the Na+/Ca2+ exchange system occurred via a toxin-stimulated Na+ entry into cardiac cells and was observed in the same range of toxin concentration which produced 45Ca2+ entry. The Na+ entry system that was activated by tessulatus toxin was insensitive to classic inhibitors of known Na+ entry systems in cardiac cells. Possible mechanisms by which tessulatus toxin induced Na+ entry into cardiac cells and contractions in smooth muscles are discussed. Tessulatus toxin is cytotoxic when used at high concentrations.