The effect of calcium on magnesium absorption was studied in the rat ileum in vivo. The increase in the luminal concentration of calcium led to a progressive decrease in magnesium absorption, which was accompanied by a parallel decrease in net sodium absorption. This calcium effect was also observed when sodium chloride was replaced by urea. However, a consistent correlation was observed between the magnitude of net magnesium absorption and the rates of net water absorption of all calcium concentrations. These findings suggest that calcium decreases magnesium absorption by a nonspecific reduction in membrane permeability to solutes that induce net water flow and are consistent with the concept that magnesium is transported by solvent "drag". The increase in the luminal concentration of calcium resulted in an increase in tissue accumulation of magnesium. This increase in tissue accumulation of magnesium was associated with a decrease in net sodium absorption and in the negativity of the transmural PD. These findings suggest an additional mechanism of magnesium transport operating independently of net water flow.