SUMMARY1. The 22Na efflux curve from the rat tail artery, at 35 0C, can be analysed as the sum of three distinct components, from 0 to 90 min of washout. After an initial diffusional component the two late exponential components Be-kBt and Ce-ket have the following values: B = 3-03 + 0-15 m-mole/kg wet wt. and C = 0-56 + 0-04; kB = 0-145 + 0-005 min-and kc = 0-015 + 0-007.2. In order to identify the cellular origin of the different compartments we compared the 22Na efflux curve from the rat tail artery with the curves obtained from whole rabbit aortal strips, rabbit aortal medial or adventitial strips; and primary cultures from rabbit aorta medial smooth muscle cells, cultures of a non-fusing muscle cell line (BC3H1), libroblasts and endothelial cells. 4. The efflux from compartment B of smooth muscle cells is inhibited by ouabain and in the absence of extracellular K+. The efflux from compartment C is inhibited only by ouabain but not by the suppression of extracellular K+.5. We propose a distribution of Na+ in smooth muscle cells in two intracellular compartments: (1) Na+ freely dissolved in the sarcoplasm, exchanging with the kinetics of compartment B and (2) a second cellular compartment which could be contained in the sarcoplasmic reticulum exchanging with the kinetics of compartment C.* To whom correspondence and reprint requests should be sent. R. P. GARA Y AND OTHERS 6. On the basis of the previous model of Na+ distribution, considering our values, and without any correction, the estimated sarcoplasmic concentration of Na+ is 9-6 mm, compatible with the direct measurements obtained in skeletal and heart muscle. The Na+ concentration in the sarcoplasmic reticulum would be 4-10 times higher than in the cytoplasm. In order to increase the accuracy of our calculations it would be necessary to account for the interdiffusion and back diffusion of Na+ between compartments. It is not possible to attain this goal at the present time.