The effect of calcium chloride infusion on the renal handling of magnesium was evaluated in thyroparathyroidectomized dogs during normal and acute reduction in glomerular filtration rate. The results show that calcium infusion increased urinary magnesium even when the filtered load of magnesium was markedly reduced. The augmented magnesium excretion was associated with an absolute increase in calcium reabsorption. These observations indicate that calcium infusion reduces the tubular reabsorption of magnesium and is consistent with the concept that these two ions share and compete for a common reabsorptive pathway in the nephron. Sodium excretion also increased during chloride infusion but a comparison of the clearances of magnesium (CMg), sodium (CNa) and calcium (CCa) indicates that CMg was more closely related to CCa than CNa when calcium chloride was infused alone. With the addition of saline infusion, CMg became more dependent on CNa