In calcium deficiency states such as chronic renal failure, 1,25-dihydroxyvitamin D3 increases calcium and magnesium absorption toward normal levels. In the present study, the ability of exogenous 1,25-dihydroxyvitamin D3 to increase calcium and magnesium absorption above normal rates in healthy subjects was investigated. Steady-state perfusion studies were performed in 30 cm segments of jejunum and ileum before and after one week of 1,25-dihydroxyvitamin D3 administration (2 micrograms per day, 10 subjects). Serum 1,25-dihydroxyvitamin D concentration increased from 25.8 +/- 2.5 pg/ml to 56.4 +/- 6.6 (mean +/- SEM, p less than 0.05). In the basal state, calcium absorption was significantly higher in the jejunum than in the ileum. Vitamin D administration resulted in a significant increase in calcium absorption which was quantitatively similar in both the jejunum and ileum. The changes in net movement were due to an increase in lumen-to-plasma flux of calcium; the plasma-to-lumen flux remained unchanged. Jejunal magnesium absorption also was enhanced by 1,25-dihydroxyvitamin D3. These studies demonstrate that in healthy persons, exogenous 1,25-dihydroxyvitamin D3 increases calcium absorption in both the jejunum and the ileum, and increases magnesium absorption in the jejunum.