SummaryA tracer dose of Mg28 (10 to 25 μc contained in 3 to 10 meq of magnesium) was administered orally to 26 subjects, and serial specimens of blood, urine and feces were assayed for radioactivity. Urinary excretion within 72 hours accounted for less than 10% of total dose of Mg28; the maximal excretion (6.3%) occurred during the first 24-hour period. Maximal concentration of radioactivity in plasma occurred at 4 hours, but the actual increase in serum magnesium was negligible. Fecal excretion within 120 hours accounted for 60 to 88% of administered dose. Low renal excretion of magnesium is believed due to poor gastrointestinal absorption of this material.