Calcium distribution was studied in the small intestine of piglets fed skim milk powder (SMP) or defatted soybean flour (DSF ) to investigate the relationship between calcium availability and its forms. Ionized calcium in duodenal and ileal digesta was measured with a selective calcium electrode that was not affected by changes in pH or sodium, potassium and magnesium concentrations, which simulated the liquid phases of digesta. Eight piglets were fed DSF-based diet or SMP-based diet for 30 d, and duodenal and ileal digesta were collected. Soluble calcium concentrations in the ileum were higher in the SMP-fed group than in the DSF-fed group. The proportion of soluble calcium in higher-molecular-weight fraction (MW > 3000) was significantly greater in the ileum than in the duodenum of the SMP group, but did not differ between these intestinal segments within the DSF group. This proportion was significantly higher in the ileum of the SMP-fed group than in that of the DSF-fed group. In the ileum, ionized calcium concentration was significantly greater in the SMP-fed group than in the DSF-fed group. These results suggest that the increase of calcium in the higher-molecular-weight fraction raises soluble calcium concentration and changes the distribution of calcium in the ileum of the SMP-fed group. The complexes of calcium with higher-molecular-weight ligands may be easily exchangeable with ionized calcium, and the increase in these calcium complexes may consequently enhance the recruitment of ionized calcium, which then can be absorbed.