M agnesium absorption was studied in the normal human jejunum and ileum by in vivo intestinal perfusion, using test solutions containing from 0 to 20 mM Mg (as MgCl2). As luminal Mg concentration was increased, the rate of absorption in the jejunum rose progressively with a tendency towards saturation at the higher concentrations. The kinetics and rates of Mg absorption in the ileum were comparable to those in the jejunum, with the exception that at higher luminal concentrations the ileal absorptive process was fully saturated. Using test solutions containing various combinations of Ca and Mg, we found that Ca had little or no influence on Mg absorption, even through Mg depressed Ca absorption to a modest extent. Patients with end-stage renal disease, who had a reduced rate of Ca absorption (presumably due to deficiency of 1,25-dihydroxycholecalciferol) were found to have a severe depression of Mg absorption. On the other hand, patients with absorptive hypercalciuria and nephrolithiasis, who had an increased rate of Ca absorption, were found to absorb Mg normally. These results suggest that Mg absorption in the human is mediated by a transport process different from that which facilitates Ca absorption, and that normal Mg absorption may be dependent on vitamin D. Our results do not establish whether or not the normal intestine can absorb Mg against an electrochemical gradient.