1. During the first 2 hr washout of ²⁴Na from rat extensor digitorum longus muscle fits a sum of two exponentials, neither of which represents loss of extracellular tracer. This implies a model with two intracellular components.

2. Results of suitably designed experiments indicate that the two components are bidirectionally connected to each other as well as to extracellular space. These results are incompatible with a model in which every fibre is homogeneous with respect to Na concentration and flux, but in which there is a distribution of these properties among fibres.

3. Results are consistent with identification of the more slowly exchanging component as sarcoplasm and the more rapidly exchanging component as sarcoplasmic reticulum (SR).

4. Parameters of the general model include six transport coefficients, two volumes, and contents of two Na pools. The number of equations is inadequate to yield unique solutions by which the values of these parameters can be calculated. However, we derive inequalities that place upper and lower limits on the parameters.

5. If the model is correct, the rate constant for Na efflux from SR to extracellular space is at least five times greater than that across sarcolemma. Under standard conditions flux (per muscle weight) from SR is at least 100 times greater than that from sarcoplasm.

6. Under standard conditions, only 2-4% of intracellular Na, or 0·5-0·9 m-equiv/kg wet wt., is in sarcoplasm, and the rest is in SR.

7. Bounds on fluid volumes of sarcoplasm and SR under standard conditions are calculated with the assumption that Na concentration in SR is the same as in extracellular space. According to the calculations, fluid volume of sarcoplasm is 0·54 ml./g wet wt. Fluid volume of SR is about 0·124 ml./g wet wt., or 14·3% of fibre volume, in agreement with Peachey's estimate (1965) of volume of SR in frog muscle.

8. Three tests are applied to the model, with the following results: (a) volume of sarcoplasm increases in hypotonic solution and decreases in hypertonic solutions, as predicted for an osmometer. Volume of SR tends to change in the opposite direction, in agreement with results of Birks & Davey (1969) from electron microscopy on frog muscle; (b) the major effect of partial substitution of external Na by Li is a reduction in Na content of SR, with no significant change in that of sarcoplasm or in volume of either component; (c) the major effect of 10^-5 M ouabain is an increase in Na content of sarcoplasm, with no demonstrable change in that of SR or in volume of either component.

9. These results support the model, particularly our identification of the slowly exchanging component as sarcoplasm, identification of the rapidly exchanging component as SR, and the assumption that Na concentration in SR is close to that in extracellular fluid.

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