The effect of readmitted Na on the Ca-depletion process in the carbachol-sensitive Ca-store was investigated using thin bundles of the K-depolarized, Na-depleted guinea-pig taenia caecum. To estimate the quantity of stored Ca, the "Ca-load Ca-release" method was employed: To load the store with Ca, 2 mM Ca was added and left (phase A), and then it was removed by washing with 2 mM EGTA-containing solution (phase B); 10(-3) M carbachol was then applied (phase C). Instead of glucose, pyruvate was used as a nutrient. Millimolar concentrations of Na inhibited the carbachol-induced contractions when Na was present during phase B. Na present during phase A also reduced the carbachol-induced contraction, but not when Na was treated during phase C. As the period of phase B was prolonged, the carbachol-induced contraction was reduced, which suggests the depletion process of stored Ca resulting from extrusion of Ca from the cell. Na present during phase B accerelated the rate of the Ca-depletion process. Li mimicked Na with regard to the promoting effect on the depletion process, and the effect of Na was not affected by 10(-4) M ouabain, suggesting that Na-Ca exchange would not be involved in the effect of Na. These results suggest that Na may be involved in the maintenance of cellular Ca-homeostasis through the stimulation of efflux of Ca from the store to the outside of the cell.