AbstractThe absorption of magnesium (Mg2+) was measured after oral administration to dogs as a sustained‐release MgCl2·6H2O OROS formulation (MgCl2) or as magnesium aspartate hydrochloride (MAH). The compounds and their respective placebos were given at five 90 min intervals in 7.5 mEq doses (sequential protocol) or in ascending daily doses of 7.5, 22.5, and 45 mEq over 3 days (ascending protocol). Urinary excretion of Mg2+over 24 hr was significantly increased compared to placebo in the dogs given sequential MgCl2or MAH and was two‐fold greater in the MgCl2‐treated group compared to the MAH group. Plasma Mg2+increased more rapidly in the MgCl2‐treated dogs but was identical in both groups 1 hr after the fifth sequential dose. Ascending daily doses of MgCl2and MAH increased urinary Mg2+and plasma Mg2+maximally 2–4 hr after each dose; the magnitude of the effect was dose‐dependent in the MgCl2group. Sequential doses of MgCl2or MAH increased urinary Ca2++excretion by 1.06 and 1 mEq/24 hr respectively while the ascending doses of 22.5 and 45 mEq MgCl2produced significantly greater urinary Ca2+excretion than the same doses of MAH. Plasma Ca2+was not changed by either Mg2+salt in either dose regimen. The data indicate that Mg2+is readily absorbed after oral administration in the form of either MgCl2or MAH, but MgCl2is absorbed more rapidly. Increased urinary excretion of Ca2+after MgCl2or MAH may reflect a displacement of tissue