The effects of low Na on the tetanic contractility after trains of twitch were investigated in frog skeletal muscle. Tetanic tension was induced by a stimulation of 100 Hz for 400 msec every 5 min and a train of twitches (150 or 250) with 1 Hz was interposed between two successive tetani. When the Na concentration was reduced to 50% by replacement with choline, each twitch constituting the positive staircase during stimulus train was augmented as compared with those in normal Ringer. The amplitude of action potential during the staircase was smaller than that in normal Ringer and was gradually decreased, suggesting that low Na solution made the release of twitch-Ca from the sarcoplasmic reticulum in twitch more effective. Although the tetanic contractility in low Na was almost the same as that in normal Ringer before applying twitch train, it was more markedly inhibited after the train. The relative size of the inhibited tetanus had a correlation with the relative size of the potentiated twitch in both normal Ringer and low Na. This tetanus inhibition after the twitch train might be due to the suppression of action potential and/or due to some inhibitory factor in the excitation-contraction (E-C) coupling process.