1. Scorpion toxin II is potent in inducing contracture and spontaneous contractions of the chick biventer cervicis muscle. In addition, this toxin induces membrane depolarization and blockade of neuromuscular transmission in this muscle preparation. The purpose of the present study is to explore the possible mechanism of actions of toxin II. 2. The muscle contracture induced by toxin II is moderately accelerated by Ca2+-free Krebs solution, delayed by high Ca2+ (10 mM), high Mg2+ (10 mM) and low Na+ (60mM) Krebs solution. Moreover, this action is inhibited slightly by d-tubocurarine and completely by either procaine or tetrodotoxin, but unaffected by beta-bungarotoxin. All these findings suggest that toxin II induces contracture mainly by increasing the Na+ permeability of the muscle membrane. 3. Spontaneous contractions induced by toxin II are abolished by Ca2+-free Krebs solution, inhibited partially by either d-tubocurarine or beta-bungarotoxin and completely by tetrodotoxin or procaine. These results suggest that toxin II induces spontaneous contractions partially by releasing acetylcholine from nerve endings and partially by increasing the Na+ permeability of the muscle membrane.