Renal absorption of calcium and magnesium occurs in the proximal tubule, and at unknown site(s) beyond this segment. In the presence of blockade of Henle’s loop by ethacrynic acid, the excretion of calcium, magnesium and sodium in response to acetazolamide as compared to that with parathyroid hormone (PTH) was examined. Addition of acetazolamide to ethacrynic acid increased calcium and sodium excretion, but did not change magnesium excretion. In contrast, PTH added to ethacrynic acid did not change the excretion of any of the ions under investigation. Analysis of calcium and magnesium excretion as a function of sodium excretion, however, suggests that ethacrynic acid inhibited the effect of PTH to augment calcium absorption, but did not alter magnesium absorption, implying that the nephron site of PTH action to augment calcium absorption is the ascending limb of Henle’s loop and that to augment magnesium absorption resides beyond Henle’s