The effect of hypomagnesemia on renal concentrating ability was assessed in rats fed diets either low in magnesium or low in magnesium and calcium for 30 days. The rats fed a low-magnesium diet became hypomagnesemic (0.26 +/−0.03 versus 1.53 +/−0.04 mEq/L in controls), hypercalcemic (5.96 +/−0.04 versus 5.22 +/−0.11 mEq/L in controls), and hypokalemic (3.1 +/−0.1 versus 4.2 +/−0.4 mEq/L in controls) with decreased muscle content of potassium. Despite being hypomagnesemic, hypercalcemic, and potassium depleted, the rats had normal renal concentration ability (2499 +/−65 versus 2415 +/−119 mOsm/kg H2O in control). Those rats fed a diet low in both magnesium and calcium became hypomagnesemic (0.41 +/−0.08 versus 1.53 +/−0.04 mEq/L in controls) but were hypocalcemic. They also had normal renal concentrating ability (2399 +/−109 versus 2415 +/−119 mOsm/kg H2O in controls). It is concluded that hypomagnesemia does not decrease renal concentrating ability in rats. Furthermore, a normal concentrating ability demonstrated in hypomagnesemic rats, in spite of hypercalcemia and potassium depletion suggests that hypomagnesemia may ameliorate the deleterious effects of hypercalcemia and/or potassium depletion on renal concentrating ability.