Dantrolene sodium (1.0 to 10.0 mug/ml) inhibited twitch tension, tetanus tension, and contracture induced by potassium and low concentration of caffeine in the frog toe muscles. The drug also inhibited rapid cooling contracture (RCC), i.e., contracture caused by rapidly cooling the muscle from room temperature to 0 degrees C after immersing it in Ringer's solution containing a subthreshold concentration of caffein. The drug also suppressed RCC in depolarized muscle fibres which were treated with high concentration of potassium. Comparing the effect of the drug on twitches augmented with nitrate and with caffeine, the former was more markedly inhibited than the latter. On the basis of these results, the possibility that the drug acts mainly by inhibiting the movement of "trigger calcium" which in turn releases calcium from the sarcoplasmic reticulum is discussed.