The apparent diffusion coefficient of Ca ions in the extracellular space of guinea pig taenia coli was estimated from model experiments to be 3.2 x 10(-6) cm2 sec -1, while the relationship between Ca concentration in the medium and tonic tension in 40 mM K-contracture was measured in the same muscle both in the presence and absence of dextran 10. On the basis of these experiments and certain assumptions, the time course of tension decline by Ca withdrawal during K-contracture was calculated. Under all dextran concentrations tested (0--15%) the calculated time course of tension decline was in good agreement with one actually observed except in a short period of time immediately after Ca withdrawal. The results suggested that Ca distribution in the extracellular space during the loss of Ca is in good agreement with the diffusion theory in cylinder, and that each muscle fiber shows its tension without delay in response to the change in Ca concentration in the vicinity of the fiber according to the tonic tension-Ca relationship mentioned above. The discrepancy between calculation and observation was partially explained on an experimental basis.