The effect of maitotoxin (MTX), the most potent marine toxin as yet known, was studied using the skeletal muscle of the larval meal worm, Tenebrio molitor. In normal saline, Tenebrio muscles responded with the spike to direct stimulation. In the saline containing tetraethylammonium (TEA) the all-or-none action potential which had characteristic plateau was elicited by membrane depolarization. When MTX (5 X 10(-9) to 10(-8) g/ml) in the TEA saline was added, the plateau of action potential was prolonged more than in the saline containing TEA alone. Furthermore, MTX lowered the threshold, so that action potentials were readily evoked in the saline containing MTX. In either case, effects, of MTX were antagonized by Co2+. These results suggest that MTX activates the voltage-dependent Ca2+ channels in the insect muscle.