1. The relationship between the [Ca](o), the [Na](o) and the strength of the contracture evoked when the [Na](o) is reduced has been investigated in isolated frog atrial trabeculae.2. The strength of the contracture varies by the [Ca](o) (2) and by 4 radical([Na](o)) over the lower tension range.3. The contracture induced by reduction of [Na](o) is not sustained, but relaxes spontaneously. The rate of this relaxation is only dependent on the [Na](o) is the presence of strophanthidin.4. After the spontaneous relaxation of an Na-free contracture, the ability of the trabecula to develop tension upon a second challenge with Na-free solution returns in about 3 min if the muscle is perfused with Na-containing fluid. This recovery process is slowed if the [Na](o) is low during the recovery period, but the recovery is hastened by electrical stimulation of the preparation or by perfusion with K-free or strophanthidin containing sodium-Ringer.5. It is suggested that the influx of Ca(2+) which induces the Na-free contracture depends on the presence of Na(+) inside the cells. When the intracellular Na concentration falls, the Ca influx falls, and the muscle relaxes as a result of the activity of an intracellular relaxing structure.