In previous microperfusion studies in the rat, increasing luminal fluid magnesium concentration from 1.5 to 6.0 mEq/l resulted in a 62% fall in proximal and a 74% fall in distal tubular osmotic water permeability. To determine whether this reduction in rat proximal tubular osmotic water permeability produced osmotic dysequilibrium between proximal tubular fluid and plasma, proximal (TF/P)osm osm were measured during isotonic saline and magnesium chloride infusion. In 11 rats, proximal (TF/P)osm was 0.99 ± 0.01 during isotonic saline and 0.98 ± 0.01 during magnesium chloride infusion; plasma magnesium concentration increased from 1.76 ± 0.12 to 6.03 ± 0.30 mEq/l. The effect of magnesium on the ability of hydropenic antidiuretic rats and dogs to reabsorb free water was examined during hypertonic mannitol and saline diuresis. In the rat free water reabsorption was not affected by hypermagnesemia. In the dog hypermagnesemia caused a reduction in free water reabsorption at low levels of solute excretion and formation of a urine hypotonic to plasma at high levels of solute excretion. The reduction in rat proximal and distal tubule osmotic water permeability is not sufficient to affect osmotic equilibration across these tubular segments. Free water reabsorption, reflecting solute reabsorption in the ascending limb of Henle's loop, is decreased by magnesium in the dog but not in the rat.