Chemically induced diabetes in the rat is associated with a number of functional abnormalities in the intestinal tract. The transport of glucose, amino acids and fatty acids are increased, whereas that of calcium and magnesium is decreased. Previous studies in calcium transport utilized in vivo perfusion and in vitro everted gut sac techniques. The present studies determined calcium uptake by the brush border membranes of controls, diabetic and diabetic rats treated with insulin or 1,25(OH)2 Vitamin D3. Calcium uptake with time was markedly decreased in diabetic rats compared to controls. Calcium uptake at 30 minutes was 4.4 +/- 0.8 and 28 +/- 0.9 nmoles/mg protein in control and diabetic rats, respectively (p less than 0.001). Kinetics of calcium uptake at 5 seconds showed a Vmax of 2 +/- 0.02 and 2.5 +/- 0.1 nmoles/mg protein (p less than 0.05) and a Km of 0.6 +/- 0.1 and 0.54 +/- 0.1 mM in diabetic and controls, respectively. Calcium uptake at 30 minutes showed a Vmax of 15.4 +/- 1.2 and 144.8 +/- 12 nmoles/mg protein (p less than 0.001) and Km values of 0.6 +/- 0.09 and 0.5 +/- 0.08 mM in diabetics and controls, respectively. 1,25(OH)2 Vitamin D3 treatment increased Vmax to 42.8 +/- 6 nmoles/mg protein/30 minutes, whereas insulin treatment increased the Vmax to 71 +/- 8 nmoles/mg protein/30 minutes. The results suggest that calcium uptake by brush border membranes is markedly decreased in diabetic brush border membranes compared to controls. 1,25(OH)2 Vitamin D3 and insulin partially corrected calcium uptake by diabetic brush border membranes.