Three groups, each of 6 normal rabbits were starved for 17 to 48 hours prior to feeding of a dilute solution of Mg28SO4. Mean urinary excretion of radioactivity in 48 hours in the 3 groups ranged between 10 and 12.5%. The high concentration of radioactivity detected in the midabdomen by external survey was traced to the fecal content of cecum and colon. Fecal excretion was increased by resumption of feeding after exhibition of Mg28, and concentration of radioactivity over the midabdomen decreased. Poor gastrointestinal absorption of magnesium accounts for its low renal excretion. Apparently, absorption from the larger intestine does not appear.