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1. The tension levels of the contractures induced in frog atrial trabeculae by reduction of the bathing Na concentration have been investigated over a wide range of [Na+]o and [Ca2+]o, making use of EGTA to buffer [Ca2+]o under 10(-5)M. 2. The relationship between tension and the quotient [Ca2+]o/[Na+]2o is only an apparent one, and holds approximately for Na concentrations as low as 1-5 mM. 3. However, for low Na concentrations tension depends on the absolute values of both [Ca2+]o and [Na+]o. 4. Extracellular Ca is the essential factor in tension production. The results indicate an apparent threshold for tension near 5 X 10(-8) M-[Ca2+]o. 5. It can be shown that, under the present experimental conditions, the Na concentration around the cells must be lower than about 0-1 mM in 'zero' Na+ solutions. 6. It is shown that the over-all process leading to tension production can be explained in terms of a reaction involving two Ca ions. 7. To account for the results, a modification of the reaction scheme put forward by Lüttgau & Niedergerke (1958) is proposed, involving a competitive action between Ca and Na ions at a free receptor site. 8. To account for the scheme, an example of a mechanism involving both internal and external Na and Ca concentrations is considered.